252 PALAEONTOLOGY OF ILLINOIS. 



fill a broad, deep, rather rounded sinus in front of the other 

 valve ; posterolateral margins also abruptly deflected to meet 

 those of the opposite valve ; beak small, rather pointed, in- 

 curved upon that of the ventral valve ; mesial sinus very broad, 

 shallow, and not distinctly defined. Dorsal valve much more 

 gibbous than the other, the most convex part being in the 

 antero-central region, thence sloping with a moderately convex 

 outline to the beak ; lateral and antero-lateral margins abruptly 

 curved to meet those of the opposite valve ; mesial elevation 

 not well defined, rather flattened, and scarcely traceable to the 

 middle. Surface of each valve ornamented by from about 

 fourteen to eighteen rather rounded, or sometimes obtusely 

 angular plications, which are defined to the beaks ; four to six 

 of these plications usually occupy the mesial sinus, and about 

 the same number the mesial elevation. Fine, obscure, concen- 

 tric stride are also seen on well preserved specimens. Length 

 of a medium sized, rather gibbous specimen, 0.90 inch; breadth, 

 0.98 inch; convexity, 0.91 inch. 



At the time we published a description of this species, under the name . 

 Rhynchonella subtrigona, we had not access to Prof. McCoy's work on the Car- 

 boniferous Fossils of Ireland, nor had we received Mr. Davidson's valuable 

 Monograph of the British Carboniferous Brachiopoda. Since seeing these 

 works, we are strongly inclined to think our shell identical with Camarophoria 

 isorynclia of McCoy. Gibbous specimens of it certainly resemble very closely 

 the figures of th species given by these authors. A large majority of the 

 specimens, however, are less gibbous, while they all have the front more flat- 

 tened at right angles to the plane of the valves, and the sinus and anterior 

 projection of the ventral valve, broader. As it is not possible to determine, 

 from the few figures yet published of the C. isorynclia, the extent of its varia- 

 tions in these characters, we do not feel quite warranted in referring our shell 

 to that species. 



All of the five other specimens we have had for comparison, afe less gibbous 

 and proportionally broader than the one we have figured, though one of them 

 is considerably larger and none of them are smaller. On grinding across the 

 beaks of one of these specimens, we find that it shows very satisfactorily the 

 internal characters of the genus Camarophoria. 



