BRACHIOPODA. 
191 
yielded somewhat to the robust forms possessing a cardinal process, which are 
referred to the genus Uncinulus. We have not been able to obtain exhibitions 
of the hinge-structure in all these numerous forms and consequently reserve an 
opinion with regard to the proper association of some of those of less common 
occurrence. It is, however, interesting to find the structure of Camarotcechia 
possessed by the extravagantly gibbous species R. ventricosa, as precisely the 
same combination of external and internal characters reappears in the later 
faunas of the Waverly group. 
In the Oriskany sandstone we meet with a number of large and ponderous 
rhynchonellids which furnish some important evidence as to the values of the 
characters upon which the classification here adopted is based. In Rhynchonella 
Barrandii, Hall, which probably attained the greatest size of any of these 
species, the median division of the hinge-plate and the septal cavity appear to 
have been always present, a cardinal process absent. In R. speciosa, Hall, and 
R. pliopleura, Conrad, the younger shells have the same cardinal structure, but 
with increased age, probably for the most part after maturity, the median pit 
becomes obscured by the deposition of testaceous matter about the bases of the 
crura until no evidence of it remains but a linear median depression. This 
extreme is attained only in old shells, and the groove indicating the line of 
union of the lateral parts of the hinge-plate is never obliterated. Thus the 
hinge-plate takes on the appearance of a single solid lobe. In the pedicle-valve 
of young shells of all these species there is, close to the apex, evidence of very 
thin dental lamellae cemented to the lateral walls of the shell. The teeth, 
however, do not rest upon these, as their extremities are not free, but both in 
this stage of growth and always afterwards they are continuous with, and rest 
upon the lateral walls of the valve, as in the genus Rhynchotrema. The grada¬ 
tional variation indicated by these shells, in characters which in other groups 
are of indicial value according to their degree of development, leads to the con¬ 
viction that the homogenity of Camarotcechia as a zoological association will 
be better assured by removing these and similar species therefrom and applying 
to them a distinctive term of subordinate value, e. g., Plethorhyncha. Among 
the species of the Upper Helderberg, Hamilton and Chemung faunas, few will 
