168 ■ PALjEONTOLOGY OF NEW YORK. 



liiunas of the later Lower Silurian, associated with Paterula, while Schizobolus 

 presents the only association of obolelloid characters known in Devonian faunas. 



KUTORGINA, SCHIZOPHOLIS, VOLBORTHIA, IPHIDEA, ACROTBETA, AcROTHELE, LlNNARS- 



SONIA aiicl DisciNOPSis belonged in a general sense to contemporaneous faunas 

 of primordial age, while Chonotreta, Schizambon and Siphonotreta are from 

 the Lower Silurian. 



We may have to seek the source whence these numerous closely allied pri- 

 mordial groups are derived, in some earlier comprehensive stock of which we now 

 have no knowledge. The ages preceding the Silurian afforded abundant time 

 for a tendency to variability to express itself; how far the apparent order of 

 development will harmonize with the actual order of succession in the subor- 

 dinate faunas of this time, must await demonstration. 



The marginal pedicle-slit has followed still another line of development. 

 The simple incision of the margin as in Paterula, becomes in Schizocrania a 

 wide fissure extending to a subcentral beak, i.e., for nearly the radius of the shell. 

 Our observations upon the embryonic stages of Orbiculoidea have shown that 

 the fissure, in early conditions of growth, is of similar character, its margins 

 straight and divergent, but subsequently uniting to form the narrow pedicle- 

 groove and tube. In Trematis, the margins of the slit are curved, but they 

 never unite at the outer extremity, while in the genus OEhlertella of the 

 Waverly group, the margins are more nearly straight, but the incision always 

 open. 



The apical accretion in the foramen of Schizocrania, the structure of which 

 has been alluded to in the preceding pages, finds its homologue in the allied 

 genera. In Trematis it is less distinctly developed than elsewhere, but in 

 GEhlertella its tripartite arrangement into a central groove for the passage of 

 the pedicle, and two strong lateral walls enforcing the same effect, brings it 

 into precise agreement with the structure of the groove in Orbiculoidea and 

 Schizotreta, save that in the former the posterior walls of the slit have united 

 at maturity. 



