INVERTEBRATE PALEONTOLOGY. 267 



rounded, thread-like, longitudinal, raised lines, which are crossed somewhat 

 obliquely upward from the inner to the outer side of the curve, by very tine, 

 regular lines of growth ; the longitudinal lines terminating abruptly a little 

 below the apex, or smaller extremity (where they number about fourteen), 

 and increasing in number by intercalation, and diminishing in size and regu- 

 larity toward the larger end, so as to become nearly obsolete on the lower 

 half of the shell. 



Diameter at smaller end, 05 inch ; diameter of a fragment 1.16 inches 

 in length, 0.15 inch ; diameter of largest fragment near aperture, 0.22 inch. 



Since describing this shell under the name of D. fragile, I have seen 

 other specimens, which show that it varies a little in thickness and some of 

 its other characters, so that there seems to be no satisfactory constant differ- 

 ences by which it can be distinguished from D. gracile of Hall and Meek. 

 Still it is possible that a more complete series of specimens from the two 

 localities may show some slight differences by which they can be separated. 



Compared with foreign species, this will, in the first place, be at once 

 distinguished from the Cretaceous D. decussation, Sowerby, by its smaller 

 size, more gradually tapering form, and finer and closer longitudinal striae ; 

 that is, making the comparison with d'Orbigny's figure of that species in the 

 Paleont. Fr., t. II, 40J, pi. 236, figs. 1-6, Sowerby's figure being very unsatis- 

 factory. It is perhaps more nearly related to D. crassulutn, Stoliczka, from 

 the Cretaceous of India, but differs in being less pointed at the immediate 

 apex, and more finely striated toward the other extremity. 



Locality and position — The type-specimens of this species were found 

 on Sage Creek, Dakota, in the upper beds of the Fort Pierre group ; and 

 those figured on our plate were found at about the same horizon on the 

 Yellowstone River. 



Genus ENTALIS, Sowerby. 



Synoti. — Antalis. Entale, Enlaliies, &c, wholly or iu part, of pre-Liunseau authors. 



lEntaliuv), Defrance (1819), Diet. Sci. Nat., XIV, 517 (= Pharetrium, Konig, 1825). 

 Entalis, Sowerby (1839), Man. Conch., 42.— Gray (1844), Synop. Brit. Mus., 58; and (1847) 

 Proceed. Zool. Soc , 158.— Stoliczka (1870), Palajont. Indica, II, 438. 



Etym.— ? 



Type. — Dentalitim cntale, Linn. 



Shell tubular, gradually tapering and more or less arcuate from the 

 aperture to the open posterior extremity ; aperture' circular, simple, without 



4 



constriction; posterior opening small, provided with a slightly projecting, 



