GASTEROPODA. 
Ill 
Surface marked by strong longitudinal or revolving stria3, which alternate in 
size, are sometimes fasciculate, and often finer and more numerous on each 
side of the dorsal band than on the lateral portions of the shell. The 
revolving striae are cancellated by finer, subequal, thread-like transverse 
striae. The dorsal band is narrow, rarely elevated or sometimes scarcely 
raised above the surface, and usually flat or slightly concave—the con¬ 
centric striae making an abrupt retral curve upon it in crossing. The 
band is likewise usually marked by two, three, or more revolving striae, 
finer than those on the sides of the shell, and sometimes quite obscure. 
In the exfoliation of the shell, and even in the best preserved specimens, the 
elevated transverse striae sometimes become obscure towards the aperture, and 
the revolving striae also becoming obsolete leave a border marked only by the 
finer striae of growth. 
The prevailing size of the shell, as seen in the calcareous shale, is about three- 
fourths of an inch in length—specimens rarely reaching an inch or an inch and 
a quarter in length, with a width of about seven-eighths to nearly an inch and 
a half. A large, well-preserved specimen, with expanded aperture, measures, 
as exposed in the stone, an inch from the back of the dorsum to the front of 
the aperture, with a transverse diameter of one inch and a half; while 
another specimen of the same width has a length of only seven-eighths of an 
inch—due probably to compression. 
Nearly all the specimens are more or less distorted by pressure, and to this 
cause is mainly due the variation in proportional length and breadth. The 
broad concentric wrinkles which mark some of the specimens, are also in great 
measure the result of the same cause. As a general character, the exposed 
part of the outer volution increases somewhat rapidly but uniformly for about 
half its length, when it expands more abruptly; this feature is shown in 
figs. 6, 12 and 14—the revolving strife either increasing in width or becoming 
fasciculate and spreading. The surface-markings present a considerable degree 
of variation, as illustrated in the figures on plate 23. 
The mesial or dorsal band varies in character, sometimes preserving only 
the arched transverse striae, or with faint indications of revolving striae, while 
