a conspicuous difference between the two. of the revolving striæ are usually preserved.

In the casts, faint impressions

The specimen figs. 20-22 is a cast of the interior, of remarkably rounded form, and showing a slight sinus on the left beneath the apex. The dorsum in fig. 22 is improperly represented as evenly convex, while there is a slight angularity visible in the specimen.

Formation and localities. In the coarse shales of the Hamilton group at Hamilton, and at Pratt's Falls in Madison county, N. Y.

PORCELLIA, LEVEILLE.

PORCELLIA HERTZERI.

PLATE XVI, FIG. 24.

Porcellia Hertzeri, HALL. Illustrations of Devonian Fossils: Gasteropoda, pl. 16. 1876.

SHELL discoid. Volutions about four, very gradually enlarging, the last one rounded on the back, becoming ventricose towards the somewhat obovate aperture; dorso-lateral margin subangular, nodose, and sloping thence to the ventral margin; umbilical depression broad and shallow; the dorsal margin preserving evidence of a deep sinus.

The finer surface-markings are unknown. The dorso-lateral margin is marked by distinct elongate nodes, and the sloping ventral sides preserve some remains of similar markings.

The only specimen of the species known to me has a length of about one inch and a quarter, with a width of one inch. It resembles in form the European P. puzo, but the elongate nodes of the dorso-lateral margin present a very characteristic distinction.

Formation and locality. In limestone of the Upper Helderberg group in Delaware, Ohio. The species has been communicated by Rev. Mr. HERTZER, of Louisville, Ky., formerly of Columbus, Ohio.

PORCELLIA NAIS.

PLATE XVI. FIGS. 27, 28; AND PLATE XXVI, FIGS. 26-28.

Gyroceras Nais, HALL. Descriptions of New Species of Fossils, etc., p. 40. 1861.

66

66

66

Porcellia 66
Porcellia 66

66

Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 68. 1862.
Ib., Explanations pl. 6, figs. 5 and 6.

Illustrations of Devonian Fossils: Gasteropoda, pl. 16. 1876.

SHELL Consisting of about four volutions, which are somewhat rapidly expanded from the apex towards the aperture; volutions subangular and nodose on the sides, and rounded on the back; the dorsum a little elevated on each side of the mesial band; the ventral side sloping abruptly into the umbilical depression. The aperture has probably been somewhat broadly subreniform.

SURFACE of the umbilical slopes of the volution marked by strong, rounded,

transverse striæ, which are sometimes bifurcated toward the side of the shell, and are crossed by revolving lines, giving it an undulated or subnodose aspect. The back of the shell, in a partial cast, is conspicuously marked by flattened revolving bands, which are crossed by less strong transverse striæ. The dorsum is marked by a narrow depressed band, on which the striæ make an abrupt retral bend, indicating the sinuosity in the margin of the aperture. About half-way from the dorsal line to the angular side of the volution, commence low elevations which become strong oblique ridges or elongate nodes, limited by the angular margin, and not passing to the ventral region nor marking the umbilical slope of the volutions.

The larger specimen, which is figured on plate 16, preserves about one volution and a half; it is imperfect towards the aperture, and the inner volutions are broken away. It has a diameter of about two inches.

A fragment, consisting of nearly three of the inner volutions, while preserving the general features already noticed, shows the dorsal side and umbilical slopes of the volutions to be cancellated by distinct elevated equal striæ, which are conspicuously nodulose at their crossing; it also shows that the entire shell consists of at least four volutions.

This is a strongly marked species, and readily distinguished from any other at present known in these formations. It is extremely rare, since, up to the present time, but two individuals are known.

Formation and localities. In the shales of the lower part of the Chemung group in Chemung county; and near Ithaca, N. Y.

The two species of BELLEROPHON, B. obsoletus and B. repertus, having been omitted from the preceding descriptions, are inserted in this place.

BELLEROPHON REPERTUS, n. sp.

PLATE XXVI, FIG. 9.

SHELL Subglobose, rounded upon the dorsum, and regularly depressed into the umbilicus. Transverse section of one volution subcircular. Body-whorl ventricose and gradually expanding. Aperture apparently not expanding beyond the regular enlargement of the body-whorl.

SURFACE cancellated by fine, regular, revolving and transverse striæ. The transverse striæ curve slightly backward over the dorsum into the mesial band. Dorsal band narrow, depressed, well-defined, and crossed by the concentric striæ which form a distinct sinus. The species is distinguished by its regular striate surface, and the narrow depressed mesial band. Formation and locality. Hamilton group, shore of Cayuga lake.

BELLEROPHON OBSOLETUS, n. sp.

PLATE XXV, FIG. 15.

SHELL short, broadly subovate or globose, ventricose. Body-whorl regularly expanding.

SURFACE marked by obscure, lamellose, concentric striæ, which make a broad retral curve over the dorsum. The only specimen observed is imperfect, and preserved as a natural mould in the rock. Its form and surface striæ differ conspicuously from the other forms of the genus here indicated. Formation and locality. From the calcareous sandstone of the Chemung group at Nichols, N. Y.

ADDENDA TO THE GASTEROPODA.

DESCRIPTIONS OF SOME NEW FORMS, AND NOTICES OF OTHERS PREVIOUSLY KNOWN.

Since the commencement of this volume, additional information has been obtained regarding some of the species described, and a few new forms have been added to the Collection. The illustrations have been made as complete as the material available at the time would permit; but each new accession indicates the occurrence of forms not yet known or described. Much of this material is too imperfect for satisfactory determination, and farther collections are required for their study and proper arrangement. The great variation in form and other characters among the Platyceride will always render their determination a matter of great difficulty, and many specimens must be left in uncertainty. A comparison of larger collections may result in showing that some of the forms which I have distinguished can be merged, for the lines of specific separation are not strongly marked, and these often become less striking and of less importance when compared among extensive collections of individuals.

The great variability of form and character among those designated as species has its maximum expression in the genus PLATYCERAS, and continues as a marked feature in PLATYOSTOMA, which, in some species, is scarcely separable from the former genus by any persistent characters. This variable or erratic character is still manifested in the species of STROPHOSTYLUS, where it is reduced to its minimum expression.

In the higher groups the means of specific determination are better marked, but among these, so large a proportion of specimens is in the condition of casts of the interior that the determination often becomes very difficult and unsatisfactory. This is essentially true of nearly all the species in the Schoharie grit, and to a considerable extent among those of the limestones above. In this condition, many specimens are left undetermined and unillustrated, such only among them having been selected as seemed important in

giving expresssion to this class of fossils as they occur in these rocks. Since, however, this is the ordinary if not universal condition of some of the species, we have no other means of illustrating the fauna, nor the student of designating the species than from the casts; and by those alone are the formations sometimes to be distinguished.*

The following species of LOXONEMA, illustrated upon plate 28, appear to be quite distinct from those described in the preceding pages.

LOXONEMA RECTISTRIATUM, n. sp.

PLATE XXVIII, FIG. 9.

SHELL elongate terete. Volutions probably twelve or more in number, moderately convex, very gradually increasing in size, the last one being scarcely more ventricose than the preceding; each volution is distinctly contracted a little below the close suture, and then expanding gives the greatest convexity near the lower third. Suture-line close. Aperture ovate, with the columella extending below.

SURFACE marked by slender, gently curving longitudinal striæ, which bend backward from the suture to the bottom of the constriction, and then continue to the base of the volution-those of the last one curving gently forward to the columellar lip. The spaces between the striæ are from once and a half to twice the width of the ridges.

This species may be distinguished from any other described in this volume by the finer longitudinal striæ, which are scarcely curved on the body of the volution, and also by the constriction of each volution just below the sutureline. The striæ are stronger on the upper volutions, gradually becoming finer and less prominent on the lower ones, though continuing distinct throughout. The specimen figured, which preserves about eight of the lower volutions, measures a little more than one inch in length.

* Where the specimens occur in the condition of casts only, the difficulties of determining the genera LOXONEMA and MURCHISONIA are often insurmountable; and among PLATYCERAS, CYCLONEMA, PLEUROTOMARIA and CALLONEMA, the forms are so nearly alike that the casts offer few characters for generic determination.