268 TWENTIETH REPORT ON THE STATE CABINET. 



and the spiral coils are muoli more lax. These features, however, might 



not he of generic importance. 



Fig. 3. 



Atrypa reticularis : 



The interior of the upper part of the dorsal valve, showing the connecting loop of 



the spires. 



In external characters, the Zygospira and its congeners differ from 

 Atrypa in the depressed dorsal valve, with a distinct fold embracing two 

 or more simple plications ; while the ventral valve bears a median fold 

 which is marked by strong simple plications, or divided only close to their 

 origin at the apex of the shell. The plications are all simple, showing no 

 tendency to bifurcate below their origin, which is at or near the apex, and 

 in this respect resembling Rhynchonella. So far as we are able to 

 determine, the teeth-sockets are smooth, and not crenulate as in Atrypa. 



In the characteristic species of the Genus Atrypa, the valves are 

 convex, without distinct median sinus or fold, except towards the front of 

 the shell. In the young shells the dorsal valve is often very depressed 

 convex, and the beak of the ventral valve extended and perforate. The 

 plications or costae are rounded and frequently bifurcated, with numerous 

 concentric lamellose striae ; differing in this respect very essentially from 

 the simple angular plications of Zygospira. These are the most obvious 

 differences between the genera, and are, I think, sufficient to distinguish 

 them from each other. I believe, moreover, that there are important 

 differences otherwise, and that a critical study of the fossils demands that 

 these forms be generically distinguished.* 



* The forms distinguished as Ccelospira are concavo-convex shells, with bifurcating 

 and lamellose plications, and without distinctive mesial fold or sinus. 



