a similar ridge; though not invariably, as the specimens, so far as observed, never occur with the poriferous face free, and the carina being extremely thin, so that in separating from the rock it might possibly be that the carina of the dissepiments, if any exist, are broken. The non-poriferous face, on different portions of the frond, presents a variety of appearances; on some portions apparently the branches have a continuous carina very thin and but slightly elevated and the dissepiments with a semi-circular carina, not connecting with the carina of the branch; on other portions the fenestrules are surrounded by thin elevations, the space between being somewhat flattened and in the wider portions having slightly elevated irregular lines and in the narrower portions pustulose.

This species can be distinguished from F. exornata by its coarser appearance as well as by the different ornamentation of the non-poriferous face of the branches.

Formation and locality. Hamilton group; Moscow, Livingston county, N. Y.

FENESTELLA SUBTORTILIS, n. sp.

Probably infundibuliform in shape, but occurring only in fragments; largest fragment observed three centimetres long and two and one-half in diameter.

Branches comparatively slender, of nearly the same width throughout their entire length; bifurcations distant; width of branches from .25 to a little more than .33 mm.; space between equal to or a little more than the width of the branches; nine branches in the space of five millimetres; where the dissepiments on opposite sides of the branches alternate, which is generally the case, the branch is regularly flexuous; on non-poriferous side the branches are moderately convex, and with a thin, slightly elevated carina running along the middle which is frequently obliterated by weathering; the carina is finely nodose, the rest of the branch is also nodose or granulose; branches wider on poriferous side, giving the appearance of being more densely arranged than on the non-poriferous side.

Dissepiments strong, as wide or wider than the branches, six in the space of five millimetres; on non-poriferous side, on a plane with or elevated slightly above the branches, rounded, carinated; carina thin, slightly elevated and connecting with the carina of the branches; on poriferous side depressed, narrower than on the non-poriferous side. Owing to the branches being widest on the poriferous side, the appearance of the fenestrules on the poriferous face varies from that of the non-poriferous side; on which side they are broadly oval or circular; length about .5 mm.; width from three-fourths to equal the length; on poriferous side they appear much narrower, the branches sometimes being nearly contiguous.

Cells in two ranges, opening directly upward; apertures minute, circular, about .20 or .16 mm. in diameter; distance apart equal to or less than the diameter of an aperture, eighteen in the space of five millimetres; margins thin, elevated; space between ranges of apertures carinated; carina at first very thin, sinuous, thickening immediately

to about .25 mm., and having on top a thin, very slightly elevated crest. This species, especially on poriferous side, has some resemblance to F. perundulata, but is a much finer frond; the non-poriferous face resembles F. curvata, but the branches are stronger, more compactly arranged, and without spines or prominent nodes; the poriferous side is very dissimilar.

Formation and locality. Hamilton group; Moscow, Livingston county, N. Y.

FENESTELLA STRATA, n. sp.

Bryozoan infundibuliform; largest fragment observed five centimetres long and three wide.

Branches moderately strong; widest on the poriferous side, where they are .5 mm. in width; on non-poriferous side about .25 mm.; extremely sinuous, forming at the sides of the branch alternating and regular convexities and concavities; the convexities of adjacent branches touching and coalescing; on poriferous side the branches are angular, having a slight keel, which is conspicuously nodose, owing to that side of the branch being the narrowest and the angular tops of the branches coalescing; the sinuosity of the branches is much greater on the nonporiferous side, forming diamond-shaped elevations; the frond presenting a reticulated appearance, and it is with great difficulty that the direction of the branches can be determined.

Dissepiments; the points of coalition or anastomosing are in width. equal to or a little more than that of the branches; four in the space of five millimetres.

Fenestrules on non-poriferous side oval, sometimes nearly circular, usually about one millimetre in length; width two-thirds to threefourths the length; the size and shape, however, are somewhat variable; on non-poriferous side appearing much smaller both as regards length and breadth; the branches on poriferous side, though sinuous, present a much straighter appearance than on the non-poriferous side. Cells in two ranges, opening directly upward or slighly laterally, minute, circular; .14 mm., or a little less, in diameter; distance apart more than the diameter of an aperture, about eighteen in the space of five millimetres; margins thin, distinctly elevated; space between the ranges of apertures carinated; carina moderately thin, elevated about .20 mm., sinuous and finely crenulate.

This species in its sinuous, anastomosing branches resembles F. inflexa, but the branches are more slender, and on the non-poriferous side it has two ranges of apertures, divided by a carina, while that species has three or more ranges without carina. In F. perundulata the frond on non-poriferous face has a much more irregular appearance, and is more decidedly anastomosing.

Formation and locality. Hamilton group; Moscow, Livingston county, N. Y.

ON THE STRUCTURE OF THE SHELL IN THE

GENUS ORTHIS.

BY JAMES HALL,

It is generally pretty well known among palæontologists, at the present time, that the genus ORTHIS, as constituted by Dalman, contains heterogeneous material; and that the species do not form the well characterized natural group sometimes claimed for them.

Leaving out of consideration the two species first named by the author of the genus, which are marked with an ?, the remaining species exhibit a considerable variety of external form and of internal marking, which characters alone are sufficient to distinguish them generally from one another.

The general aspect of the shells constituting the genus, as described by its author and extended by subsequent writers, is a sub-circular or sub-quadrate form; valves sometimes nearly equally convex, while in other examples one valve may be flat or concave. This latter feature may affect either the ventral or the dorsal valve. Both valves are furnished with an area, though this character is often but slightly developed in the dorsal valve; the opposite valve is furnished with wider area and open triangular fissure for the passage of a pedicil. The hinge line is straight, usually shorter than the width of the shell. The surface is striated or plicated, and the general aspect of nearly all the forms is so similar that they have been grouped together, generally, and by the best authors, without hesitation.

The most conspicuous external difference is between a finely striated, and a coarsely plicate surface. These differences are often accompanied by another distinguishing feature. The coarsely plicate forms, among the American species, are usually what are termed resupinate shells; the dorsal valve being the more convex and the ventral valve flat or concave and sometimes sinuate in front, but still carrying its conspicuous area and foramen. On further examination we find that many striated species are resupinate, or have the dorsal valve the more convex. An examination of the interior of the shell in all these forms shows that the muscular impression in the ventral valve is strongly defined, distinctly bilobate, limited at the margins by a strong ridge or elevated lamella, usually interrupted or non-continuous in the front. (These forms are chiefly of lower or middle Silurian in their geological range.)

On comparing other forms of the genus where the valves are nearly equal, or where the shell is plano-convex, the more convex valve is the [Sen. Doc. No. 53.]

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ventral. There are also resupinate forms which are closely allied to them; but, as a rule, the forms with finely striated surface, subequivalve or plano-convex, have the ventral valve the more convex; and the muscular impression is flabelliform with its margins lobed, and more or less distinctly limited by an elevation of the interior substance of the shell.

The resupinate forms which are more closely allied to those with flabelliform ventral muscular impressions, have the corresponding muscular imprint more strongly defined and less distinctly lobed at the margins than in the forms just noticed.

These are the most obvious distinctions among the prevailing forms of the genus ORTHIS as constituted by Dalman.

The Orthis (Platystrophia biforata) is, in some degree, an exception to all the forms above mentioned, having both valves very convex, the surface strongly plicated, with a mesial fold and sinus, as in SPIRIFERA. In its muscular areas it resembles the resupinate forms of ORTHIS of the lower Silurian rocks, often presenting an abnormal thickening of the shell around the muscular area of the ventral valve.

The Orthis biloba (Dicælosia biloba of King) of the upper Silurian rock also presents a departure from the typical forms of Orthis, but preserves the similar muscular system.

Before undertaking a revision of the materials constituting the genus ORTHIS, it has seemed desirable to ascertain whether the variations in form, surface ornamentation, or character of muscular impression, is associated with any difference in the shell-structure. For this purpose, cuttings, prepared for microscopic examination, have been made from many species, and the result has proved that all the resupinate lower and middle Silurian forms, whether plicate or finely striate, are fibrous shells, with the ventral' muscular impression small and strongly limited. They are essentially either free from punctæ in any form, or with a few scattered pustuliform pores. The finely striate sub-equivalve or plano-convex forms with flabelliform muscular impressions, have the shell punctate in lines, or radiating belts, corresponding to the rays of the shell, with an intermediate fibrous texture. The character of the punctæ, the strength and comparative width of the punctate bands, vary with the different species.

In the finely striated, resupinate forms of the Lower Helderberg, Hamilton and Chemung groups, with the smaller and more distinctly limited flabelliform ventral muscular areas, the punctate character is very marked, often occupying almost the entire surface, and the lines of the radii are shown only by a more crowded condition of the punctæ. The resupinate species here referred to are quite different in their outline and general form from those of the lower rocks, being for the most part rotund forms with the cardinal extremities rounded. These species are easily recognized, and readily distinguished from those of the preceding group by their external form alone.

The numerous species which have been already studied in their microscopic shell-structure are naturally separated into three distinct groups which may be of generic value.

The first group includes the coarsely plicate forms, with extended cardinal angles; the valves resupinate or normal in their relations; the test is coarsely fibrous, and usually without punctæ, although some

STRUCTURE OF SHELL IN THE GENUS ORTHIS.

75

species occasionally show a few large scattered pores or ducts near the front of the shell.

Professor King has proposed the name PLATYSTROPHIA for Orthis biforata, and this species in its fibrous and non-punctate texture, may be taken as characteristic of the first group, although there are some features, especially in the form of the shell and also in the muscular impressions, which do not in every respect agree with other members. With our present knowledge, we may include in this group the following species:

Platystrophia biforata, Trenton and Hudson River groups.

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tricenaria, 66

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This list will be greatly extended as soon as the shell-structure of allied species can be studied. At present only those species are included which have been studied under the microscope.

The second group embraces forms which are usually regarded as typical species of the genus ORTHIS. The shell-structure characterizing this group may be described as finely fibrous, with distinct rows of punctæ coming out along the summit of the radii; the rows of punctæ are simple, or double in some species (O. Clytie), but usually there are several rows to each ray.

The following species of this group have been microscopically studied in numerous specimens:

Orthis testudinaria, Trenton and Hudson River groups.

The third group, consisting of O. multistriata of the Lower Helderberg group, O. Lowensis of the Hamilton group, O. Tulliensis of the Tully Limestone, and O. impressa of the Chemung group, is highly punctate, with a fine fibrous texture of the shell-substance. In the great number of the punctæ and for the most part their uniform character, together with their arrangement, these forms of the Orthida resemble species of Terebratula, Cyrtina, etc. The name Schizophoria, King, may be adopted for this latter group of species.

The accompanying illustrations (plates 3 and 4) will serve to give a clear idea of the microscopic characters presented in the shell-structure of specimens in each of the three groups indicated.

The preparation of the shell sections and the photographs of these for the lithographer have been made by Mr. C. E. Beecher of the State Museum.

*Not Orthis flabella of Sowerly.