308 REPORT ON THE STATE CABINET. 



between the genera still more close. In Zygospira, the species are more 

 obliquely directed towards the centre of the dorsal valve than in Atrypa ; 

 the loop is attached much lower down on the limb of the spire, and is 

 J^;^3 more direct, as is shown in fig. 3 



(interior of the upper part of the 

 dorsal valve of Atrypa reticularis), 

 and the spiral coils are much more 

 las. These features, however, might 

 not be of generic importance. 

 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 costse are rounded and frequently bifurcated, with numerous 

 concentric lamellose striaB ; 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 latoellose 

 plications, and "without distinctive mesial fold or sinus. 



