384 GEOLOGY OF THE BLACK HILLS. 



Mr. Meek, in his description of the genu.s Syncyclonkma, which is 

 founded upon this species, does not mention the existence of a cartilage pit, 

 nor give the form of nui.scuhir imprints. The specimen in hand, whicli is 

 an internal cast in blackish limestone, shows the cartilage pit very dis- 

 tinctly and the muscular imprint quite plain. It is possible, however, that 

 we may have mistaken the nature of the latter, being misled by the small 

 size of the .shell, but the structure is exactly like that of a muscular scar, 

 although its position may be quite unusual. From the form of the external 

 border of the anterior ear we should suppose the notch was intended for the 

 passage of a byssus. 



Formation and locality. — In limestone of the Cretaceous, at the forks of 

 the Cheyenne River, Black Hills. 



HETEROMYARIA. 



AVICULID^. 

 Genus PTERIA Scopoli. 



PTEEIA LIXGUIFORMIS. 



Plate 7, tig. 2, 3. 



Avicula linguiformw Evaus & Shumard, Proc. Acad. Nat. Sci. Phil., vol. 7, p. 1C3. 

 Avicula linguiformis (E. & S.) M, & H., Proc. Acad. Nat. Sci. Phil. Nov. 1856, p. 21. 

 Pterin Ungnlformh (E. & S.) Meek, Smithsonian Clieck List Inv, Foss., p. 9. 

 rteria linguiformis (E. & S. sj).) M., Pal. U. S. Geol. Surv. Terr., p. 32, PI. 16, fig. 1. 



The examples of this species in the collection are all more or less 

 exfoliated and imperfect, and therefore do not present the natural surface 

 of the shell, so that we obtain but little else than the general form. The 

 shells are obliquely subovate or obscurely rhomboid in outline, very ine- 

 quilateral and inequivalve ; hinge line straight, nearly or quite as long as 

 the body of the shell below, but projecting anteriorly and short behind. 

 Anterior wing of the left valve of moderate size, acutely triangular, and 

 pointed at the extremity ; posterior wing long, the posterior margin nearly 

 at right angles to the hinge line for a short distance below, and then 

 directed obliquel}' backwards to the extremity of the shell ; beak large and 



