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 leatures 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 

 Palaeozoic faunas are variants of the well-known horizon-marker, Rhynchonella 

 (Atri/pa) cuboides, Sowerby ; namely, R. venusiula, Hall, of the TuUy 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. venusiula, 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 H. Emmonsi. See Palseontology of the 

 Eureka District, p. 157. 



