110 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA. Lbull.106. 



DONACID^E. 

 Genus DONAX Linmeus. 



DONAX CUNEATA 11. Sp. 



PI. xxv, Fig. 1. 



Shell transversely elongate-triangular, moderately thick, somewhat 

 compressed, especially toward the front ; anterior end very greatly pro- 

 duced, narrowly rounded at the extremity and slightly curved upward, 

 the dorsal margin being gently concave and the base somewhat convex 

 in outline; posterior end abruptly truncate about at right angles to the 

 longer axis of the shell j the truncate portion bordered by the sharp 

 prominent vertical umbonal ridge extending from the beak to the pos- 

 tero-basal angle; beak rather prominent, directed slightly forward, 

 nearly or quite terminal (at the posterior end). 



Surface apparently smooth and the free margins not eremite within. 



The hinge has not been exposed sufficiently for figuring, but its 

 structure can be seen fairly well in the type, which is a right valve. 

 There are two approximate cardinal teeth under the beak, and both 

 the anterior and posterior lateral teeth are well developed, the former 

 being elongate and located a considerable distance in front of the beak, 

 while the latter is shorter and is near the cardinal teeth. 



Length, 27 mm ; height at posterior end, 16 mm ; convexity of single 

 valve at posterior end, 45 mm . 



The type of this species is a right valve from which most of the shell 

 has been eroded. A smaller sandstone cast from another locality and 

 a higher horizon lias the same outline and probably belongs to the same 

 species. With such limited and imperfect material it is of course 

 impossible to say positively whether the generic reference above given 

 is correct, but the form and the internal features, so far as they are 

 known, agree very well with some living species of Donax. If it really 

 is a Donax it is one of the oldest known species. The oldest previously 

 described American form that has been referred to the genus is D. lata 

 Gabb, from the Tejon group of California. Stoliczka figures a specimen 

 which he says seems to be a true Donax from the upper part of the Cre- 

 taceous of southern India. The same author states that other described 

 Cretaceous species probably do not belong to this genus. 



I do not know any fossil form hitherto described with which this 

 species need be compared. 



From the species described below it differs in that its anterior end is 

 proportionally longer, more slender, and more narrowly rounded in front, 

 while its posterior end is much shorter. 



Locality and position. — The type is from coal-bearing sandstone at 

 Bear River city, Wyoming, where it is associated with Inoceramus 

 labiatus, Pugnellus fusiformis, Cardium paivperculum, etc. The other 



