144 



Genus ATHYKIS. (McCoy.) 



This genus, as originally described by McCoy and D'Orbigny, consists 

 of a large and important assemblage of species ranging from the Middle 

 Silurian up to the Permian inclusive. It appears to be capable of sub- 

 division, and accordingly several generic names have been proposed for 

 the various sub-groups. The arrangement that I follow is that published 

 by Davidson in his Introduction to the classification of the Brachiopoda. 

 According to this, ATHYRIS stands for that group of imperforate species 

 which has A. tumida for the type, and is precisely equal to MERISTELLA 

 (Hall), proposed in 1861. SPIRIGERA includes the perforated group 

 with S. concentrica for the typical form. 



If Professor Hall's suggestions be followed, then SPIRIGERA must be 

 suppressed altogether and ATHYRIS take its place, and thus stand as the 

 generic name of a group of shells for which it is decidedly inappropriate. 

 I cannot see the least reason whatever for this change, except that it 

 would make a vacancy for Prof. Hall's genus MERISTELLA. If we follow 

 Davidson, there can be no misnomer, for both ATHYRIS and SPIRIGERA 

 will stand for groups for which they are in no way inappropriate, and no 

 injustice will be done to either McCoy or D'Orbigny. 



ATHYRIS UMBONATA. (N. sp.) 



Fig. 121. Fig. 122. 



Fig. 121. Athyris umbonata. a, dorsal, and b, side view. 

 122. j$. Prinstana. a, dorsal, and b, side view. 



Description. Elongate ovate ; sides usually forming a continuous gen- 

 tle curve from the umbo of the ventral valve to the front margin, some- 

 times a little angulated at the hinge extremities; front margin either 

 rounded or with a small portion in the middle straight. Ventral valve 

 strongly convex, forming a continuous curve from beak to base, spiral in 

 the upper half, gently curved in the lower. Umbo large, rounded, very 

 prominent. Beak abruptly incurved down to the umbo of the dorsal 



