266 PALJSOi^TOLOGY. 



well preserved, and in some cases strongly separated from the body of the 

 valve; sides of the valve a little straightened; front rounded on the lateral 

 portions, but strongly emarginate in the center by the rather strong, some- 

 times abruptly depressed, mesial sinus, which extends from near the beak to 

 the front of the shell; beak moderately strong, extending beyond the hinge- 

 line and incurved. Dorsal valve nearly flat over the central area, with a 

 slight concavity in the region of the beak, the margins abruptly geniculated 

 to conform to the curvature of the opposite valve. The mesial constriction 

 is as strongly marked, but a little wider than that of the ventral valve, and 

 extends nearly to the beak. 



Surface of the shell marked by rather even, rounded striae of moderate 

 strength, except near the front of the shell, where they become indistinctly 

 fasciculate or blended; also, on the rostral half of the shell by numerous, 

 distinct, closely arranged, concentric, undulating wrinkles, but which seldom 

 extend beyond the point of greatest geniculation. A few scattered spines 

 mark the front and sides of the shell, while several stronger ones are observ- 

 able on the cardinal auriculations. On the dorsal valve, the concentric 

 wrinkles are more distinct, and occupy the entire flattened area of the valve, 

 while the radiating strise are found to be more strongly bifurcating. 



The specimens bear a very close resemblance to those from Burlington, 

 Iowa, in their general form and characters. There are a few unimportant 

 differences noticed, but not such as can be considered of specific importance. 

 There is also considerable resemblance to Prod, mesialis Hall (Greol. Iowa, 

 p. 636, plate 19, fig. 2); but that shell is much wider in proportion to the 

 length than any of those from the more western locality. 



Formation and locality. — In dark-blue limestone of Lower Carboniferous 

 age, north of Snowstorm Hill, Dry Canon, Oquirrh Mountains, Utah. Col- 

 lected by J. E. Clayton. 



Peoductus ljevicostus 1 



Plate 5, figs. 7-8. 



Productus Icevicostus White ?, Jour. Bost. Soc. Nat. Hist., 1860, p. 230, 



Shell below the medium size, subtriangular in general outline, rapidly 

 increasing in width from the small, narrow, and rather pointed Jbeak to near 



