166 PALAEONTOLOGY OF NEW YORK. 
The immediate variations from the obolelloid stock, may be expressed as 
follows: 
Schmidtia-Paterula-Sehizobolus->. 
in the substance of the shell, begins a series of variations which eventuate in 
Siphonotreta and Orbiculoidea. The association of this feature with the 
distinctive characters of the obolelloid stock has been observed only in the 
genera Schizobolus and Paterula, 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 
