87 



Genus Athyris. Mcc oy . 



Athyris, McCoy. Carb. foss. Ireland 1844. 

 Etymology : a, without; thuris, a small door, in allusion to the absence of a deltidium or door. 



Shell variable in shape ; valves unequally convex, with or without a mesial 

 sinus and fold ; articulated by teeth and sockets. Beak of ventral valve in- 

 curved, usually overlying and in contact with beak of the dorsal valve, and 

 perforated by a foramen, or, when old, the foramen fully concealed. In the 

 interior of ventral valve the dental plates are fixed to and along the sides of a 

 longitudinal prominence or convex arch- shaped place, which extends to less 

 than a third of the length of the shell, with its narrow end fitting into the 

 extremity of the beak, and its lateral diverging edges to bottom of valve. The 

 interior of the dorsal valve is partly divided by a large, deep, longitudinal 

 septum, which extends from the extremity of the umbo to about two-thirds of 

 the length of the shell, supporting at its origin the hinge-plate, which is 

 divided into two portions by a narrow, gradually widening channel. To the 

 socket ridges are affixed the spiral cones, the extremities of which are directed 

 towards the lateral margins of the shell ; on either side of the septum are seen 

 two muscular scars formed by the adductor. 



Athyris vittata. HALL. 



Plate XVI., figu-es 25-32. 



Athyris vittata, Hall. Thirteenth Eep. on State Cab., p. 891860. 

 Compare Athyris concentrica ; A. spiriferoides. 

 Athyris vittata, Hall. Pal. N. Y., Vol. IV., page 289, plate 461867. 



Shell sub-circular or sub- quadrate ; gibbous ; hinge-line short, with cardinal 

 extremities rounded ; front conspicuously sinuate. 



Ventral valve gibbous above, more convex than dorsal valve ; umbo promi- 

 nent ; the beak incurved and truncated in the plane of the longitudinal axis 

 by a round foramen ; curving very abruptly to the cardinal and lateral mar- 

 gins ; the center, marked by a mesial sinus which extends nearly or quite to 

 beak ; it is shallow, deepening and widening towards the front ; its margins 

 are not defined but rounded and coalesce with the surface of the valve ; it has 

 a considerable basal extension, which is abruptly bent upwards and fits into 

 a corresponding indentation of the other valve. Dorsal valve a little less gib- 

 bous than the ventral one ; sides regularly curving from the umbo ; about the 

 middle of the valve an elevation commences, which increases towards the front, 

 where it is of considerable height, thus forming a conspicuous mesial fold ; the 

 beak is strongly incurved into the other valve below its beak, 



