152 TWENTIETH REPORT ON THE STATE CABINET. 



The observations on tlie Genera Athyris and Meristella are as 

 follows : 



GENUS ATHYRIS (M'Coy). 



The Genus Athyris was established in 1844 by Prof. M'Coy, upon 

 certain species separated from the Terebratul^ ; and when restricted 

 according to the original types of that author, includes a very natural 

 group of shells, but which nevertheless possess many external features in 

 common with the later established Genera Merista and Meristella, 

 and from which the species are distinguished by important internal 

 characters. 



The shells of the genus are variable in form, being suborbicular, trans- 

 verse or elongate, subglobose or depressed, and sometimes subangular. The 

 typical species are depressed suborbicular. The structure of the shell is 

 fibrous ; the surface in most species is strongly marked by concentric striae, 

 and some of them are strongly lamellose, with the lamellae becoming fim- 

 briate or pectinate. In this aspect, as well as in general form, these shells 

 have a resemblance to some of the Spirifers with short area and rounded 

 cardinal extremities. The surfaces are sometimes obscurely radiatingly 

 striate ; but these striae are usually subordinate to the concentric striae or 

 lines of growth, and it may be doubtful v/hether any well authenticated 

 species of the genus has conspicuous radiating striae or costge. 



The apex of the ventral valve is usually or perhaps always perforated by 

 a rounded foramen, the lower side of which is formed by the umbo of the 

 opposite valve. When the valves are separated, this foramen communicates 

 with a triangular space which opens into the main cavity of the valve. 

 This triangular fissure, which in older shells is usually occupied by the 

 beak of the opposite valve, has at some time during the animal's life been 

 closed by deltidial plates. 



The dorsal valve is furnished with a strong cardinal process, the centre 

 of which is often depressed and spoon-shaped, but sometimes thickened and 

 convex. The crura proceed from each side of this plate anteriorly ; while 

 the posterior and lateral margins become more or less thickened, or eleva- 

 ted 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 muscular 

 imprints are marked upon the shell, and upon the cast, by a narrow elon- 

 gate 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 space 

 outside of the muscular scars is papillose or papillose-striate, and often 

 beautifully marked b^? vascular impressions. The spires are complicated by 

 intermediate lamellae. 



The European species of Athyris {A. pectinifera, A. roissyi and A- coU' 

 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 Euro- 

 pean forms. In the determination of our species, however, and their rela- 

 tions with Meristella, it became necessary to make a careful study of all 

 the internal characters upon which generic and specific distinctions might 



