2IO NEW YORK STATE MUSEUM 



terior basal margin^ and above this the surface of the posterior wing 

 is sHghtly concave; beaks incurved, hardly raised above the hinge 

 line; posterior end more sharply rounded than anterior; surface with 

 concentric striae. 



Found in the Clinton lenses on the Rome, Watertown and Og- 

 densburg railroad. 



Class GASTROPODA Cuvier 



The gastropods, or snails^ are mollusks with a distinct head, a 

 muscular foot, and a mantle consisting of a single lobe. They are 

 terrestrial, marine, 'or fresh-water animals, and are commonly pro- 

 tected by a conic or spirally coiled shell, which is secreted by the 

 mantle. The apical part of the shell usually consists of a simple 

 coiled embryonic shell, or protoconch. Succeeding this is the shell 

 proper, which, when coiled comprises few or many whorls, the later 

 overlapping the earlier ones to a greater or less extent. The suture 

 at the junction of the overlapping whorls may be faintly or strongly 

 impressed. The whorls may coil closely, forming a compact central 

 colinnella; or they may be loosely coiled, leaving a hollow columella, 

 opening below in the umbilicus. The body whorl opens in the 

 aperture, the rim or peristome of which consists of an outer and an 

 inner, or columellar lip. The peristome is complete when both inner 

 and outer lip are present, and incomplete when the place of the inner 

 lip is taken by the preceding whorl. In a great many species the 

 peristome is notched anteriorly, or produced into a straight, or more 

 or less flexed canal. A posterior notch is also frequently found. 

 The columellar lip and, in its absence, the columella, may be smooth 

 or furnished with one or more plications. Similarly, the outer lip 

 may be smooth on its inner side or furnished with plications or 

 lirae. Among the external features of importance are the transverse 

 lines of growth, which mark the successive increments; varices or 

 rows of spines, parallel to the lines of growth, and marking periodic 

 resting stages during the growth of the shell; and revolving longi- 

 tudinal lines or ridges, which may be uniform or alternating, or may 

 show a gradation in size. When transverse and longitudinal lines 

 cross each other, a reticulated surface ornamentation is produced; 

 and, when the shell is covered by an epidermis, or periostracum, hair- 

 like spines not infrequently arise from the points of crossing. In 

 Pleurotomaria and related gastropods, a siphonal notch occurs in 

 the outer lip, and its progressive closure from behind leaves a 

 marked revolving band, commonly visible only on the body whorl. 



Many species, specially of marine gastropods, secrete a horny or 

 calcareous operculum, which is attached to the foot, and closes the 

 aperture of the shell when the animal is withdrawn. This is seldom 

 preserved in a fossil state. 



