180 PALEONTOLOGY OF NEW YORK. 



The evidence leaves little room for doubt that the combination of characters 

 forming the rhynchonelloid type of structure deviated at an early age from 

 the same stock whence Orthis has been derived. The earliest " Rhynchonel- 

 las " of which we know the interior, are not Rhynchonellas in any true sense, 

 but properly connecting morphological phases between Orthis and Rhyncho- 

 NELLA, inceptive stages of the fuller development attained in later faunas. 



In this aspect of the subject it seems preferable to consider the palaeozoic 

 Rhynchonellas essentially in a chronological order, thereby leading up to the 

 later types of structure, and thus following the natural course of development 

 and variation so far as the material in hand permits. 



At the outset it will be necessary to indicate the very primitive structure 

 obtaining in some of the earliest species, and in order to distinguish these in- 

 ceptive forms it will be necessary to introduce, as a new division, the 



Genus PROTORHYNCHA, gen. nov. 



PLATE LVI. 



1847. Atrypa, Hall. Palaeontology of New York, vol. i, p. 21, jil. iv (bis), fig-. 5. 

 1862. Porainhonites, Billi.ngs. Pal.Tozoic Fossils, vol. i, p. 140, tigs. 117 a-^. 



Shells biconvex, with a low, ill-defined fold and sinus on brachial and pedicle- 

 valves respectively. Pedicle-valve with a false cardinal area defined by ridges 

 diverging from the beak. Pedicle-passage triangular, rarely showing any trace 

 of deltidial plates. Teeth very small, supported by thin lamellae which rest 

 upon the bottom of the valve and are not adnascent to the lateral walls of the 

 shell. In the brachial valve the dental sockets are small; the hinge-plate con- 

 sists of two minute discrete processes, the surfaces of which are slightly inclined 

 toward each other. These were the bases of the brachial supports but show 

 no points of attachment to the crura ; they are separated by a triangular inci- 

 sion extending to the bottom of the valve. There is no cardinal process nor 

 median septum in the brachial valve, and no trace of muscular scars in either 

 valve. 



Type, Atrypa dubia, Hall. Chazy limestone.* 



* It should be observed that these details of structure have been derived from specimens obtained fi'om 

 the gorge of the Kentucky river, at High Bridge, Kentucky. 



