GASTEROPODA. m 



Surface marked by strong longitudinal or revolving striae, 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 stria? 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 

 fics. 6, 12 and 14 — the revolving striae 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 



