200 
PALEONTOLOGY OF NEW YORK. 
frequently this is obscured to such a degree as to be observable only at the 
summit of the stout, erect process, as in R. vellicata, but'in the other species its 
duplicate character is usually retained. Figures are given in this volume, 
showing the various forms of this process from a bidentate condition very sug¬ 
gestive of its appearance in the leptaenoid shells, to a condition in which the 
parts are firmly coalesced into a simple process. These various conditions are, 
in a certain sense, developmental, but are also features of specific value, though 
it should be observed that in old shells, where there is a tendency to extrava¬ 
gant secretion, the process is thickened, as are also the median septum and in¬ 
ternal walls of the valves. The term Uncinulus, embracing these shells, forms 
a very compact association, which, in American faunas, seems to leave its last 
trace in the R. speciosa, Hall, of the Oriskany sandstone, of which mention has 
already been made in the discussion of Plethorhyncha ; a shell which, in a 
senile condition, shows a tendency to the formation of a cardinal process. 
There is good reason to believe that this peculiar combination of characters 
was derived from the Rhynchotrema of the Lower Silurian, the shells having 
somewhat the same external aspect, while on the interior the unsupported teeth 
and linear cardinal process of Rhynchotrema point to this conclusion. At the 
same time it is worthy of remark that in the latter genus the cardinal process 
lying between the parts of the divided hinge-plate, seems to be a remnant of 
the median septum, while this apophysis in Uncinulus has evidently been 
formed by the subapical coalescence of the lateral components of that plate. 
The last of the strongly subcuboidal species to appear in the American 
PaliBOZoic faunas are variants of the well-known horizon-marker, Rhynchonella 
{Atnjpa) cuboides, Sowerby ; namely, R. venusiula, Hall, of the Tully limestone, 
R. Emmonsi, Hall and Whitfield, from the upper Devonian of the White Pine 
District of Nevada, and R. intermedia, Barris, from the upper Devonian of Iowa.* 
Rhynchonella cuboides, in European faunas, is indicial of upper Devonian age. 
Its representative in the Tully limestone, R. venustula, is associated with a fauna 
composed almost exclusively of the species of the Hamilton shales. 
* Mr. C. D. Walcott regards the last named a synonym for M. Emmonsi. See Palseontology of the 
Eureka District, p. 157. 
