184 MISSISSIPPIAN BRACH1OPODA 



bers. Internally the teeth are supported by a pair of short dental plates 

 which become obsolete anterior to the articulation of the hinge. 



Brachial valve more convex than the pedicle, the greatest depth anterior 

 to the middle and sometimes close to the anterior margin, from the point 

 of greatest convexity the surface curves more or less gently to the poste- 

 rior extremity, transversely the surface is rather regularly convex but it 

 curves a little more abruptly as it approaches the lateral margins ; mesial 

 fold obsolete posteriorly and scarcely or not at all differentiated on the 

 more or less abrupt anterior slope of the valve ; the beak incurved be- 

 neath that of the opposite valve ; plications similar in character and num- 

 ber to those of the opposite valve. Internally the hinge-plate is divided 

 to its base and the cardinal process is lacking, at the extreme posterior 

 apex of the valve a median septum is present, which is divided above to 

 support the two limbs of the hinge-plate forming a crural cavity, this 



FIG. 6. A series of five cross-sections of the rostral portion of the shell of 

 Camarotcechia grosvenori (X 2 1 /&), showing the dental lamellae in the pedicle 

 valve and the short median septum and small crural cavity in the brachial 

 valve. 



cavity disappears posterior to the articulation of the hinge and the median 

 septum is rapidly reduced in height, although it frequently persists as a 

 slight median ridge to the middle of the valve. 



Aside from the plications the surface of both valves is nearly or quite 

 smooth except in fully grown shells which have a few, distinct, parallel 

 lines of growth close to the margin. 



Remarks. This species occurs commonly in the Salem limestone fauna 

 in Indiana, but in the Illinois localities it is a rare shell. It resembles 

 somewhat closely some of the variations of C. mutata, and in most large 

 collections of the latter species a few individuals may usually be selected 

 which by themselves might be referred to C. grosvenori. Typical repre- 

 sentatives of the two species, however, are quite distinct. C. grosvenori 

 is a smaller species, much more globular in form, with a much less well 

 developed sinus in the pedicle valve, and with finer and less angular pli- 

 cations. Many and perhaps a majority of examples of C. grosvenori are 

 longer than wide, while all but a very few specimens of C. mutata are 

 wider than long. 



Most recent writers have referred this species to the genus Pugnax, 

 but a careful examination of the internal characters shows such reference 

 to be incorrect. With the small crural cavity between the bases of the 

 divided hinge-plate, supported by a median septum, the species must be 

 excluded from the genus Pugnax. 



