210 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Mactra (Cymbophora) alta, M. & H. 



Plate 37, figs. 2, it, b. 

 Maclra alta, Meek and Hayden (1856), Proceed, Acad. Nat. Sci. Philad., VIII, 271. 



Shell thin, triangular, compressed ; dorsal slopes diverging from the 

 beaks at an angle of about 110° ; extremities subangular, or abruplly 

 rounded ; base forming a broad semi-oval curve ; beaks much elevated, small 

 or pointed, incurved, and very nearly central ; lunule and escutcheon lance- 

 oval, the latter being larger than the other and bounded by a more distinctly 

 angular ridge; surfice marked only by moderately distinct lines of growth. 

 Hinge of right valve thick and strong, with the spoon-shaped cartilage-cavity 

 large, and provided with very prominent margins; cardinal tooth small (not 

 well shown in fig. 2, a), quite closely connected with the hinge-margin just 

 in front of the cartilage-cavity ; lateral teeth strong, and very distinctly 

 double. Hinge of left valve unknown. 



Length, about 2.62 inches; height, 2.15 inches; convexity, 1.15 inches. 



Young individuals of this species, judging from the curve of the lines of 

 growth on mature specimens, must resemble M. formosa in form ; but even 

 then the ridges bounding the lunule and escutcheon (particularly the latter) 

 are more distinct. The beaks are also more elevated, and, in old or fully- 

 developed individuals, the valves are decidedly more compressed, and pro- 

 portionally much higher; it also attains a larger size than M. formosa, and 

 lias a thicker and stronger hinge. 



So far as I have been able to make out the characters of its hinge from 

 the left valve (fig. 2, a, pi. 37), this shell seems to agree well with Cymbophora, 

 Grabb. Its cartilage-cavity appears to be connected with the slit for the recep- 

 tion of the ligament above ; but this seems to be due to the breaking away 

 of the raised upper margin of the pit. Mr. Gabb's figure of the hinge of 

 Cymbophora does not show any marginal slit for the ligament ; but this may 

 have been accidentally omitted. 



Locality and position. — Mouth of Judith River, on the Missouri; and at 

 several localities in Colorado, west of Greely. 



