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SOME ABNORMAL AND PATHOLOGIC FORMS OF FRESH-WATER
SHELLS FROM THE VICINITY OF ALBANY, NEW YORK.
BY CHARLES E. BEECHER.
Monstrosities among fresh-water shells are not infrequent and are interesting as illustrative of the cause of natural or accidental deformity. A large proportion of abnormal or pathologic forms is found in exposed situations, where the shells are subject to varying conditions of water and materials brought by currents or otherwise. The annual draining and cleaning of the canals renders the contained organisms liable to many accidents. It is likewise found that in the vicinity of a ford or watering-place for cattle, many of the uniones bear the marks of injury. It is, while the animal is repairing these injuries and adapting itself to changing conditions of water and deposits, that most of the malformations in its shell are produced, and it is quite seldom that a shell is found which has been deformed by the atrophy or hypertrophy of any of the animal organs. These malformations are occasionally transmitted and their degree is often augmented by the action of the law of accelerated heredity, as applied to the mollusca by Professor Alpheus Hyatt.*
It is convenient to consider abnormalities as natural or accidental. Natural changes are usually produced by the action of gravitation, adaptation to modified habitats or by changes in the forms of the organs. The effects of gravitation are noticed mainly in those univalves which live at or near the surface of the water and, therefore, necessarily carry the weight of the shell at a disadvantage.
Accidental deformities are always the accompaniment of an attempt by the animal to repair injuries which it has received. If the form of the shell has been altered, the animal will accommodate itself to this alteration ; and, on the contrary, if permanentinjury or malformation has been produced in the soft parts of the animal, the accreting test will gradually adjust itself to this change in those parts.
* The Genesis of the Tertiary Species at Steinheim, by Alpheus Hyatt; page 27, Anniversary Memoirs of the Boston Society of Natural History, 1880.
One of the most noticeable and interesting examples of a departure from normal conditions is sinistrality. With some genera and species (Partula, Achatinella, Bulimus, etc.) the dextral or sinistral shells occur indifferently. Thus, from a sinistral specimen of Campeloma, Raf. (Melantho, Bodw.) were taken two sinistral fry. The remaining twenty-five were dextral. Also, some of the fry of dextral individuals are very often sinistral. In other genera only certain (supra) species are sinistral, and again in some entire genera (Physa, Clausilia, etc.) this is a constant feature. Many genera and species have not yet furnished a single example. Two remarkable sinistral forms are given in the present paper. Several others, among our land and fresh-water shells, are known from the State of New York, but not in the vicinity of Albany. Individuals are found among our uniones which have the cardinal and lateral teeth interchanged in the valves, thus giving to the right valve the form and number of teeth belonging to the left. This kind of sinistrality is of unusual occurrence, and has been rarely noticed.
Upon the authority of Professor R. Ellsworth Call, I am able to cite the following species in which he has observed the above reversion of teeth : Unio complanatus, Mohawk, N. Y.; U. rubiginosus, Des Moines, Iowa, and U. cahawbensis, Cahawba river, Alabama. He has also had the kindness to make several valuable suggestions and corrections in the subject matter of the present paper.
DESCRIPTION OF SPECIMENS.
Plate I, figs. 6–8. Figure 6 represents a specimen with an unusually expanded aperture. The first thickening of the labrum is immediately succeeded by another thickening of the margin, which is also flexed outward and produces the enlargement.
The second specimen, figure 7, exhibits the tendency of the outer volution to become free. The suture is very deeply impressed nearly to the columella, and the aperture is much shortened.
These two specimens exhibit natural departures, while figure 8 represents an accidental deformity, in which the margin of the aperture is deeply excavate and the lower part of the labrum is sinuate.
PLANORBIS EXACUTUS, Say.
Plate I, figs. 1–3. The examination of a large series of specimens from the vicinity of Albany shows that this species frequently departs from its normal form. Individuals with expanded and variously modified apertures are not uncommon and one sinistral example has been detected.
Figure 1 represents an individual in which the upper side of the labrum is expanded.
Figure 2 represents an individual in which the entire aperture is inflated, especially on the lower side.
The sinistral specimen (figure 3) has lost nearly all the testaceous characters belonging to the species and is a monstrosity in every particular. It is impossible to determine from external evidence whether it is a case of true sinistrality or one of inverted growth. The volutions are of equal convexity on either side and the obliquity of the aperture is not determinative. The specimen was found in a locality abounding only in this species, and the three specimens here described were selected from among several thousand others, about two per cent of which show some departure from normality, principally in variations in the form of the aperture and elevation of the spire and in intermittent growth.
VALVATA TRICARINATA, Say.
Plate I, fig. 9. The volutions of the specimen are free except at the apex. This variation in this species has been recorded by several observers and is not extremely rare, although this is the only specimen which has been found in the vicinity of Albany.
GILLIA ALTILIS, Lea.
Plate I, fig. 5. A very remarkable biflexed individual. The shell, for a considerable period of its growth, equal to the formation of the three initial volutions, is dextral and of the usual form. The spiral then changes its direction; the apex becomes partially inverted and the last volution is sinistral. This is the only example of a heterospiral growth that is known to me and cannot be satisfactorily accounted for from the appearance of the shell alone. An examination of the anatomy of the animal might have revealed the cause of this reversion of growth. The initial point of the operculum being nearer to the apex of the shell, indicates that the growth was inverted during the formation of the last volution, and suggests, as a possible explanation, the action of gravitation on an animal too weak to hold the shell on its dorsum.
SOMATOGYRUS SUBGLOBOSUS, Say.
Plate I, fig. 4. The carination of the volutions and narrowing of the upper part of the aperture is often observed in individuals of this species. The specimen figured is an extreme development in these particulars, and presents a marked departure from the usual form.
UNIO PRESSUS, Lea.
Plate I, figs. 10–12. Figure 10 represents the left side of a specimen which is unusually alate at the post-cardinal extremity. The outer zone of growth slopes rapidly to the pallial margin and is marked by the absence of the colored radii. In the specimen the body of the shell is of a dark-green color, while the last annulus of growth is yellow and presents a strong contrast with the remaining portion of the shell.
The next figure (figure 11) represents a specimen which received an injury during the early growth of the shell. The margin of the valve is flexed and there is a broad mesial depression in the right valve extending from the umbo to the margin. In the left valve the conditions are reversed, the depression in the right valve being represented by a corresponding plication.
Figure 12 shows a left valve with the anterior portion narrow and auriculate, the umbo oblique and the wing much reduced. The teeth of this specimen are also much modified ; in the left valve there is a single continuous elevated tooth which is sinuate anteriorly to represent the cardinal teeth. In the right valve the teeth are quite rudimentary and the strong cardinal ridge of the opposite valve projects into the rostral cavity.
UNIO CARIOSUS, Say.
Plate I, fig. 13. The figure represents a small gibbous female with the anterior end unusually narrowed. Individuals of a similar character are not unusual, although they are seldom as ventricose as in the present instance.
UNIO NASUTUS, Say.
Plate II, fig. 1. A female showing a row of seven vertical plications on the zone of growth adjacent to the last, with obscure traces of similar plications made at an earlier period of development.
UNIO COMPLANATUS, Solander. "
Plate II, figs. 2-6. Figure 2 represents a specimen similar to the preceding, but with more numerous and stronger vertical plications. The shell in these species is normally smooth and we must seek for an explanation of the cause of the plications in the soft parts of the animal, as they are evidently not due to accidental causes. From the examination of a number of individuals presenting these plications in various degrees of prominence, and from the inspection of the living animal, it is evident that these abnormal features are produced by the rapid growth of the shell over the gills while they are distended with fry. Unio osbeckii, a species from China, is classed with the plicate forms in Lea's Synopsis of the Unionidæ, but the plications do not seem to be a constant characteristic of the species. The plications are not always present and, when they do occur, they are usually obscure and similar in position and expression to those specimens of U. nasutus and U. complanatus here presented and probably have a like significance.
Figure 3 represents the right valve of a specimen modified by accidental deformity. The umbo is nearly central, and the upper anterior portion of the valve is flattened and deeply sulcate.
The next specimen (figs. 4, 5) is a very elongate cylindrical form with an excessively thickened pallial margin.
The last individual to be noted (fig. 6) is an apparently normal form, as no marks of accidental or natural deformity can be detected. It was found associated with numerous specimens of U. complanatus, and is here referred to this species, although seemingly presenting marked specific differences. The outline is regularly elliptical, and the prominent beak is situated just anterior to the middle of the length. The cardinal teeth are elongate, and the lateral tooth is short and oblique — characters which do not belong to U. complanatus. Should it ultimately prove of a distinct species, it would be of a form hitherto unknown to this locality.
Specimens similar to the preceding briefly noted forms are often overlooked or considered as unimportant by inany collectors ; but to a student of morphological variation and possible specific change, they are extremely interesting. After numerous accidental and natural changes have been illustrated and described, embracing many genera and species, it will be possible to generalize important biological facts relating to the classification of species and manner of growth of the organisms.