389 PALiBONTOLOGY OF NEW-YORK. 



Geivijs Atiiyris (McCoy). 



Mhyru, McCor, 1844. Spirigera, D'Obbiokt, 1847. 



The Genus Athyris was established in 1844 by Prof, McCoy, upon 

 certain species separated from the Terebratul.e ; 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, 

 transverse or elongate, subglobose or depressed, and sometimes suban- 

 gular. 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 

 lamella) becoming fimbriate 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 indistinctly radiatingly striated ; but these striae are usually 

 subordinate to the concentric striae or lines of growth ; and it may be 

 doubtful whether any well authenticated species of the genus has con- 

 spicuous radiating striae or costae. 



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 commu- 

 nicates with a triangular space, which opens into the main cavity of the 

 valve. Tbis 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 thick- 



