BRACHIOPODA. 191 



yielded somewhat to the robust forms possessing a cardinal process, which are 

 referred to the genus Uncinulds. 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 CAMAROTtECHrA 

 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 Camarot(echia 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 



