LOWER SILURIAN LAMELLIBRANCHS. 681 



Genus PYRENOMCEUS, Hall. 



Pyrenomceus, Hall, 1852, Pal. N. Y., vol. ii, p. 87. 



I refer two species provisionally to this little known genus. They 

 agree in all respects with the published characters of P. cuneatus, Hall, 

 the type of the genus, except that the anterior muscular scar is not 

 strong, being on the contrary even less distinct than the posterior one. 

 So far as the muscular impressions can be made out they seem to be as 

 in the most of the Nuculidcz. Still, I have no idea that they really 

 belong to that family of shells, the hinge being thin and almost certainly 

 without denicles, while the test is very delicate and polished externally. 

 Stoliczka, in his great work on the Cretaceous Pelecypoda of Southern 

 India," places the genus with the Solenomyidcs. This arrangement seems 

 to me entirely unwarranted, the short, nuculoid form of the shell being 

 totally unlike the elongated form so strictly adhered to by the typical 

 members of that family. All other authors who have had occasion to 

 refer to Pyrenomceus have placed the genus more or less doubtfully with 

 the Nuculidtz, and here I would leave it till something definite is learned 

 of its hinge, when I believe we will have reason for the erection of a 

 new family. 



Pyrenomceus decipiens, n. sp. 



Plate 51. Figs. 7 and 8. 



Shell from 12 mm. to 15 mm. long, 9.8 mm. to 11.5 mm. high, nucu- 

 loid in shape, narrowest posteriorly, strongly convex, with prominently 

 rounded umbones situated between one-third and one-fourth of the entire 

 length from the anterior extremity. Antero-cardinal region slightly com- 

 pressed and subangular in outline ; upper half of anterior margin nearly 

 vertical, base regularly convex; posterior margin oblique, scarcely trun- 

 cate, sharply rounded below. Umbonal ridge rounded, inconspicuous; 

 an obscurely defined line or narrow ridge traverses the middle of the 

 cardinal slope posteriorly from the beak. Test very thin, the surface pol- 

 ished and marked by very fine concentric lines, of which the strongest 

 onhy are visible to the unassisted eye. A small heart-shaped ligamental 

 area or lunette in front of the beaks. Hinge very thin, as far as ob- 

 served, without teeth. Anterior muscular scar and pallial line not shown 

 by any of the casts of the interior seen; we may therefore assume that 

 they are very faint; posterior scar scarcely defined, rather large, occupy- 

 ing the greater part of the post-cardinal slope. 



The casts of this shell look very much like testiferous examples of 

 an undescribed associated species of Ctenodonta. The latter is usually 

 identified by Cincinnati collectors with Hall's Tellinomya levata. Testif- 

 erous specimens of the two species are no more likely to be confused than 

 are casts of the interior. In the first case the surface of the shell is not 



