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 P R 0 T 0 RH YN C H A, gen. nov. 
PLATE LVI. 
1847. Atrypa, Hall. Palseontology of New York, vol. i, p. 21, pi. iv (bis), fig. 5. 
1862. Poramhonites, Billings. Palseozoic Fossils, vol. i, p. 140, figs. 117 a-g. 
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 from 
the gorge of the Kentucky river, at High Bridge, Kentucky. 
