144 



73-75, and illustrations on p. 93) pointed out the distinc- 

 tion between Athyris=Spirigera and Meristella. This 

 difference is everywhere clear and unmistakeable, in the 

 external lamellose surface of the one, and the almost smooth 

 character of the other. The muscular impressions of the 

 ventral valve of Athyris are at once distinguishable from 

 those of Meristella ; as may be seen on comparison of 

 figs. 35 and 36 with figs. 29 and 30 



Fig. 35. Fig. 36. 



In the dorsal valve, the muscular impressions differ from 

 Meristella; the hinge plate is of somewhat different 

 character, and the median septum is scarcely developed. 



