NATURAL SCIENCES OP rHILADELPHIA. 147 



tremity. Lower valve rather deep ; ligament area small and well defined ; 

 beaks somewhat pointed at the extremity, most frequently turned abruptly to 

 the left; umbonial region sometimes a little distorted by the cicatrix of at- 

 tachment. Upper valve flat, or a little concave ; beak truncated at the ex- 

 tremity. Surface smooth, or only marked by very small, faint concentric wrinkles 

 of growth. Length about 3 inches ; breadth 1-50 to 2 inches. 



Locality and position. Mouth of Judith River, in sandstone of formation No. 1 ? 

 of general section. 



OSTREA TRANSIiCCIDA. 



Shell small, thin and translucent, arcuate, attached by the under side of the 

 lower valve in the umbonial region ; border having faint indications of three or 

 four broad undefined plications ; beaks small, more or less distinctly curved to 

 the left ; lower valve convex, rather deep ; ligament area very small and 

 triangular, sometimes linear and abruptly curved to the left ; margins on 

 each side of the beak regularly and rather distinctly crenulated ; muscular impres- 

 sion ovate, small, very near the left border, indistinct ; upper valve nearly flat, 

 or slightly concave near the middle, sometimes distinctly alate on the left side 

 at the beak, regularly and closely rounded on the other side; cardinal end 

 transversely truncate ; beak not extending beyond the truncated cardinal edge, 

 but curving round to the left within the margin ; surface nearly smooth, or 

 marked only by faint subimbricating lines of growth. Length about 1 inch; 

 breadth -53 inch ; depth of lower valve '30 inch. 



This species resembles 0. larva of Lamarck, and is the shell referred by ns 

 with doubt to that species in the catalogue of Nebraska Cretaceous and Ter- 

 tiary fossils given in our last paper. A comparison of better specimens in the 

 late collections, with authentic specimens of 0. larva proves them to be quite 

 distinct. Our shell never being near so distinctly plicated round the border as 

 that species, nor is it auriculate on both sides of the beak as in 0. larva. Its 

 mode of attachment is likewise altogether different, being more like 0. crenulata 

 of Tuomey, in that respect; it also sometimes resembles the latter shell in the 

 crenulate character of its borders on each side of the beaks, but differs in want- 

 ing the closely plaited squaniose ridges mentioned by Prof. Tuomey. 



Locality and position. Long Lake on the Missouri, in the upper part of No. 5 

 of the section. 



HeMIASTER ? HUMPHREYSANUS. 



Shell broad cordate or subcircular, rather depressed, greatest elevation near 

 the posterior, declining very slightly towards the front; broadest a little in 

 advance of the middle, rather flat below ; oral aperture, not large, transversely 

 oval, arcuate, with convex side towards the front, located about one-fourth the 

 length of the shell from the anterior end ; vent small and of an oval form ; api- 

 cal disc not more than one-third the length of the shell in advance of the 

 posterior end : genital pores four, rather large ; dorsal ambulacra, excepting 

 the odd one, distinctly petialoid, very unequal ; anterior or odd one long, 

 lanceolate, and extending nearly to the margin, located in a deep rounded 

 groove which passes over the front to the mouth, giving an emarginate cha- 

 racter to the anterior end, having about thirty-three pairs of pores in each 

 series ; anterior lateral ambulacra flexures a little broader than, and not quite so 

 long as, the odd one, placed in distinct grooves, and having about thirty-five or 

 six pairs of pores in each series ; posterior ambulacra very small, not much more 

 than one- third as long as the anterior laterals, oval in form, and having about 

 eighteen pairs of pores in each series, not deeply excavated ; inter-ambulacral 

 spaces prominent, anterior two rather pinched up ; surface unknown. Lent'th 

 1-21 inch; height -81 inch ; breadth 1*20 inch. 



Not having seen specimens showing the surface markings of this species, it is 

 with some doubt we refer it to the genus Uemiaster, the absence of the shell 



1857.] 



