CONTRIBUTIONS TO PALEONTOLOGY. 



155 



The Atrypa modesta ( loc. cit.) is the typical species of this genus, the 

 interior of which is shown in the accompanying illustrations. Both figures 

 are much enlarged : fig. 1 represents the dorsal valve removed, and shows 

 the loop as actually seen in the specimen, extending across the centres of 

 the spires. Fig. 2 represents another specimen with the ventral valve re- 

 moved, and the spires lying in the dorsal valve ; the loop being shown in the 

 position actually seen in this specimen*. 



Fia. 1. 



Fm. 2. 



* By grinding or polishing the surfaces, and macerating in acid, the internal spires 

 are frequently shown in a satisfactory manner. 



