GENUS ATHYRIS. 
283 
ened and convex. The crura proceed from each side of this plate ante¬ 
riorly ; while the posterior and lateral margins become more or less 
thickened, or elevated into ridges bordering the teeth-sockets. 
The muscular area is somewhat variable in form ; but in the ventral 
valve it is oval or ovate, more or less flabelliform. The occlusor mus¬ 
cular imprints are marked upon the shell, and upon the cast, by a narrow- 
elongate scar; while the divaricator muscles occupy a wider space on 
each side, and are usually' strongly striated. In the dorsal valve the 
muscular area is narrow, and often divided by a low longitudinal crest 
or septum. The area 'outside of the muscular scars is papillose or papil¬ 
lose-striate, and often beautifully marked by vascular impressions. The 
spires are complicated by intermediate lamellae. 
The European species of Athyris (A. pedinifera, A. roissyi and A.con- 
centrica ), as shown by Woodward, Davidson and others, have the spires 
complicated by accessary lamellae, which, rising from the connecting loop, 
are intercalated between the first and second turns of the normal 
lamellae, and have their extremities free. 
Although the presence of spires has been long known in the American 
species, I believe no one has hitherto shown their analogy with the 
European forms. In the determination of our species, however, and their 
relations with Meristella, it became necessary to make a careful study 
of all the internal characters upon which generic and specific distinctions 
might be founded; but since all the specimens were essentially solid, it' 
has been a labor of no little difficulty to determine accurately the true 
character of these internal appendages. By carefully cutting down speci¬ 
mens of A . spiriferaides, which is very similar to the A\ concentrica of 
Europe, the disposition of the spiral lamellae has been found as shown 
in the following diagram, which represents only the central portion 
of the spirals to the end of the first volution. 
