GENUS ATHTRIS. 983 



ened and convex. The crura proceed from each side of this plate ante- 

 riorly ; while the posterior and lateral margins become more or les3 

 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. pectinifera, 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- 

 Inens of A.'spiriferoides, 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. 



