CAMAROTCECHIA 179 



CAMAROTCECHIA TUTA (Miller) 

 Plate XXIV, Figs. 9-28 



1881. Rhynchonella tuta Miller, Jour. Cinn. Soc. Nat. Hist., vol. 4, p. 315, 

 pi. 7, figs. 11-llb. 



Description. Shell small, length and width subequal or somewhat 

 longer than wide, subovate in outline, the greatest width near or a little 

 anterior to the mid-length, the postero-lateral margins nearly straight, 

 sometimes a little concave or a little convex, the antero-lateral and an- 

 terior margins rounded or the anterior margin somewhat truncated. The 

 dimensions of a nearly complete specimen from the type locality are: 

 length of pedicle valve 6.4 mm., length of brachial valve 5.7 mm., greatest 

 width 6.3 mm., thickness 4.5 mm., width of sinus in front 4.1 mm. The 

 dimensions of a specimen from the Burlington white chert at Louisiana, 

 Missouri, are : length of pedicle valve 7.3 mm., length of brachial valve 

 6.7 mm., greatest width 7.5 mm., thickness 5 mm., width of sinus in front 

 4.3 mm. 



Pedicle valve convex in the umbonal region, the surface curving ab- 

 ruptly to the postero-lateral margins, gently convex from the umbonal 

 region to the antero-lateral margins and arched in a subsemicircular curve 

 along the mesial line from beak to front, with the posterior curvature 

 a little more abrupt than the anterior: mesial sinus originating near the 

 mid-length of the valve, nearly flat in the bottom transversely, produced 

 anteriorly in a rather broadly rounded or subrectangular lingual ex- 

 tension; beak small, pointed, strongly incurved, coming nearly in con- 

 tact with the umbo of the opposite valve, the delthyrium and deltidial 

 plates obscured by the incurvature of the beak ; plications simple, originat- 

 ing at the beak, rounded, becoming subangular towards the front, four 

 usually if not always occupying the sinus, with about five upon each 

 lateral slope which become successively smaller towards the cardinal ex- 

 tremities ; concentric markings obscure or obsolete. Internally the dental 

 plates are delicate and of moderate length, and are not widely divergent 

 anteriorly from the beak. 



Brachial valve more strongly convex than the pedicle, the greatest 

 convexity anterior to the middle, sometimes near the front margin, the 

 surface arched from the beak to the front along the median line, usually 

 with the curvature more pronounced posteriorly, the curvature more 

 convex to the antero-lateral margins, becoming abrupt to the postero- 

 lateral margins ; mesial fold originating near the mid-length of the valve, 

 not strongly differentiated from the general curvature of the valve 

 except close to the anterior margin, in the umbonal region a shallow, 

 longitudinal, broadly subangular, mesial depression is present, which is 

 deepest at the beak and becomes obsolete before the point of origin 



