64 



The external sculpture eoii«ists of very tine uikI close set concentric 

 strise. The markings on the interior can only be traced, and that very 

 oI)scurely, on the right valve of one of the casts. In this valve there 

 are indications of three cardinal teeth, which divei-ge widely from above 

 downwards. The anterior tooth is oblique and almost longitudinal; the 

 centre one is shoi-t, triangular, and nearly ti-ansverse to the hinge line; 

 while the posterior tooth is long, obli(^uo and dii-ected backwards. The 

 pallial iniju'essions seem altogether obliterated. 



In an average specimen, the length is rather more than fifteen lines ; 

 the height, in the centre, is thirteen lines; and the thickness through 

 the valves, six lines. 



INine or ten specimens were obtained by Mr. Eichardson, three of 

 which are quite perfect, with the shell preserved on both valves. The 

 outline of the species is very variable, some specimens being neai-ly 

 ovate while others are suljtrigonal. C. suhtrlgona is a flatter shell with a 

 more triangular form than C. Deiveyi of Meek and Hayden,^^ and 

 C. orhiculata of Hall and Meek f has the posterior side more broadly ana 

 evenly rounded. The figure on Plate VIII. represents a variety in 

 which the anterior and posterior ends are not nearly so much pointed as 

 usual. 



Callista. (?) (8p. undt.) 

 Plate IX., fig. 11. 



Shell compressed, but rather tumid in the middle ; very inequilateral ; 

 outline elongate ovate ; length about a fourth greater than the height; 

 test very thin. The beaks are situated very near to the anterior end, 

 but are not quite terminal ; the}' are small and jjoint forwards, but their 

 apices do not rise above the highest level of the hinge border. There 

 seems to be no lunule proper, and the escutcheon is a narrowly lanceolate 

 deep groove, which is bounded on each side by a shai-p ridge. Behind 

 the beaks the hinge line is almost straight, and its downward curve is 

 very gentle ; the posterior end is narrowly, and the basal margin 

 broadly rounded. Immediately beloAV the beaks, in front, a short and 

 concave lunular declivitj^ extends to a little above the middle of the 



* "Proceedings of the Academy of Natural Sciences of Philadelphia, 1856," page 83. "Report on 

 the Invertebrate Cretaceous and Tertiary Fossils of the Upper Missouri Country." By F. B. Meek. 

 Washington : 1876. Pages iS2-3, Plate XVn., figs. 15, a, b, c, d, e. 



t "Memoirs of the American Academy of Arts and Sciences, Cambridge." Vol. V New Series. 

 Pages 382-3, Plate I., fig. 7. " Report on the Invertebrate Cretaceous and Tertiary Fossils of the Upper 

 Missouri Country/' &e., pages 186-7, Plate V., figs. 2, a, b, c. 



