CAMAROTCECHIA 177 



cardinal margin, the last two or sometimes three being almost obsolete. 

 Internally, the hinge-teeth are supported by short, dental lamellae which 

 scarcely extend beyond the articulation of the hinge. 



Brachial valve gibbous, the greatest depth at or near the anterior 

 margin, the surface sloping from the anterior margin to the beak along 

 the median line with a convex curvature which is a little flattened in 

 the middle portion, the umbonal region flattened transversely with a dis- 

 tinct longitudinal median depression which continues anteriorly for about 

 one-fourth the length of the valve ; the median fold not differentiated in 

 the posterior half of the valve, flattened or gently convex on top and 

 only moderately elevated above the lateral slopes anteriorly ; lateral slopes 

 strongly convex longitudinally and curving abruptly to the margin of 

 the shell laterally ; the beak strongly incurved beneath that of the opposite 

 valve; plications similar in form and number to those of the opposite 

 valve, those of the lateral slopes strongly arched longitudinally. In- 

 ternally no cardinal process is present, the hinge-plate is divided and is 

 supported towards the beak by a divided median septum which forms 

 a crural cavity, the support of the hinge-plate by the divided median 

 septum is discontinued posterior to the articulation of the hinge, but the 

 free septum continues for about one-third the length of the valve, gradually 

 becoming lower until it disappears. 



A few obscure concentric lines of growth are sometimes present, but 

 the shell is usually smooth aside from the plications. 



Remarks. This little shell is a common form in the fauna of the 

 Chouteau limestone and may be easily recognized by its size and angular 

 plications. It differs from Rhynchoporaf cooperensis of the same fauna 

 in its smaller size, relatively narrower shell and smaller number of plica- 

 tions on the fold and sinus. 



Horizon. Chouteau limestone. 



CAMAROTCECHIA ELEGANTULA Rowley 

 Plate XXIV, Figs. 1-8 



1900. CamarotcBcliia elegantula Rowley, Am. GeoL, vol. 25, p. 264, pi. 5, 



figs. 44-47. 



Description. Shell sm&ll, broader than long, the postero-lateral margins 

 nearly straight or slightly concave and meeting at the beak in an angle 

 of from 90 to 110 degrees, the antero-lateral and anterior margins rounded. 

 The dimensions of three nearly complete specimens are : length of pedicle 

 valve 7.5 mm., 9.5 mm., and 8.5 mm. ; length of brachial valve 7 mm., 8.5 

 mm., and 7.6 mm. ; greatest width 8.3 mm., 11.6 mm. and 9.4 mm. ; thick- 

 ness 5.1 mm., 6.7 mm., and 6 mm. ; width of sinus in front 6 mm., 8 mm., 

 and 5.9 mm. 



