DIELASMOIDES 255 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle, convexly curved from front to beak with the curvature 

 progressively greater posteriorly, the surface only gently convex trans- 

 versely in the posterior two-thirds of the valve, except towards the 

 postero-lateral margins where it is abruptly curved towards the opposite 

 valve and becomes inflected towards the cardinal extremities; towards 

 the front of the valve, about one-third of its length from the anterior 

 margin, two shallow, broadly rounded, longitudinal depressions or sinuses 

 originate, which lie symmetrically on each side of a low, broadly rounded 

 mesial fold which, however, does not rise above the general surface of 

 the valve outside the longitudinal sinuses; the beak strongly incurved 

 so that the delthyrium is entirely hidden by the contact with the opposite 

 valve, the foramen large and encroaching wholly upon the umbonal 

 portion of the valve. Internally the dental plates are well developed and 

 extend anteriorly from the beak for about one-sixth or one-fifth the 

 length of the valve. 



Brachial valve subequally convex with the pedicle, the greatest con- 

 vexity posterior to the middle, the curvature of the surface along the 

 median line is gently convex with the curvature more abrupt posteriorly, 

 transversely the surface is somewhat narrowly rounded in the mesial 

 portion and then slopes with a gently convex curvature to the lateral 

 margins, the curvature becoming more abrupt posteriorly ; mesial portion 

 of the valve in its anterior one-fourth, depressed in a short, rather broad 

 and shallow mesial sinus, the bounding folds on either side scarcely 

 differentiated from the general curvature of the valve ; the beak pointed 

 and incurved beneath that of the opposite valve. Internally, the socket 

 plates lie in nearly the plane of the valve posteriorly, and are supported 

 at their inner margins by a pair of lamellae which extend obliquely to 

 the floor of the valve to which they are joined on either side of the 

 median line, the crural bases being an anterior extension of the inner 

 edges of the socket plates ; median septum absent. 



Remarks. This species is peculiar among our loop-bearing shells in 

 having the bisinuate configuration of the pedicle valve towards the front, 

 and the short median sinus in the brachial valve. This configuration is 

 sometimes obscure, especially in smaller or immature shells, but in full 

 grown specimens it is probably always present in some degree, although 

 more pronounced in some individuals than in others. In its general form, 

 aside from its bisinuate configuration, the species most closely resembles 

 some species of Dielasma, especially such a species as D. siibspatulata, 

 although the greatest width of the shell is not so far forward as in that 

 species. 



Locality. Patrick's Quarry, Middle Fabius river, Lewis Co., Mo. 



