252 UNITED STATES GEOLOGICAL SURVEY OF TEE TERRITORIES. 



The only specimen yet found is a cast apparently of the interior, but it, 

 may be of the exterior. It consists of the two valves, lying partly open in 

 the matrix, in their natural juxtaposition with relation to each other. It 

 shows no traces of the muscular or pallial impressions, and presents no marks 

 or indentations from which anything can be inferred in regard to the nature 

 of the hinge; consequently, it has been only doubtfully referred to the genus 

 Pharella, though it agrees more nearly in form with that than any of the 

 described genera. It is certainly not a Solen. 



Locality and •position. — Dakota group of the Upper Missouri Cretaceous 

 series ; mouth of Vermilion River, Nebraska. 



Genus LEPTOSOLEN, Conrad. 



Synon. — Siliquaria (sn.), Conrad (1858), Trans. Acad. Nat. Sci. Philad., Ill, 324 (not Sebum. ). 

 Lepiosolcn, Conrad (1867), Am. Jour. Couch., Ill, 15 ; and ib., 188. 



Etym. — Af7T7-Hf, thin ; and Solen. 

 Type. — Siliqua biplicata, Conrad. 



Shell transversely elongated, with upper and lower margins straight and 

 parallel; extremities rounded and gaping, or the posterior sometimes sub- 

 truncated ; beaks not near terminal; hinge with one small tooth in the right 

 valve, projecting at right angles inward from the cardinal margin, with its 

 anterior side rounded, its posterior truncated, and its free end sharply cuneate ; 

 interior of each valve provided with a prominent narrow rib, extending from 

 the beaks immediately behind the tooth, more or less directly downward, 

 becoming more obscure near the middle of the valves, and dying out before 

 reaching their ventral margins. 



Mr. Conrad remarks that "this genus is very nearly allied to Solena, 

 Browne; but having obtained a more complete view of its interior since the 

 generic character was published, I find that the anterior muscular impression 

 is chiefly anterior to, though its posterior end nearly covers, the rib, which 

 is prominent in the umbonal region, truncated behind, sloping anteriorly, and 

 situated behind the line of the apex. The upper margin of the muscular 

 impression is on a line with the anterior hinge-line. In Solena, the tooth is 

 narrowest on the hinge-plate, and there is a small pit before it; but, in Lep- 

 tosolen, the tooth is broadest on the hinge-plate, and tapers to a very acute 

 edge, which expands in the direction of the shell's [longer?] diameter." 



The species yet known to belong to this genus are from rocks of Creta- 

 ceous age. Leguminaria Moreana, d'Orbigny, from the Cretaceous of France 



