166 



PALEONTOLOGY OF NEW YORK. 



The immediate variations from the obolelloid stock, may be expressed as 

 follows : 



Schmidtia ■ 



Paterula • 



Schizobolus 



-> 



Obolus 



Elkania - 



- Dinobolus 



With the inception of a marginal pedicle-aperture taking the form of a slit 

 in the substance of the shell, begins a series of variations which eventuate in 

 SiPHONOTRETA and Okbiculoidea. The association of this feature with the 

 distinctive characters of the obolelloid stock has been observed only in the 

 genera Schizobolus and Paterdla, more definitely in the former. Its 

 assumption appears to have been accompanied directly by an elevation 

 of the posterior margin into a vertical, somewhat specialized area, suggesting 

 the true cardinal area of the articulates. Thus, in the genera Kutorgina and 

 its representative in the " Obolus beds " of the Salt-Range of India, ScHizo- 

 PHOLis, the subapical posterior slope, often incurved in Kutorgina, is divided 

 for its entire height by a triangular opening. In Volborthia, Iphidea, and 

 AcROTRETA, essentially the same features are present, save that the pedicle-slit 

 is represented by a ridge in Volborthia and Iphidea, and a furrow in Acrotreta, 

 terminating in an aperture at the apex of the shell (Volborthia ?). 



It has already been observed, that the explanation of these pedicle-characters 

 and their apparent differences, may be found in the mode of growth of the shell. 

 In Kutorgina and Schizopholis, the mantle of the shell or the pedicle itself has 

 failed to secrete testaceous matter behind the aperture as growth advanced, thus 



