FOSSILS OF THE LOWER CARBONIFEROUS. 267 



the front of the shell, which is broadly rounded and destitute of any mesial 

 depression; hinge-line short, scarcely more than half as wide as the body of 

 the shell. Ventral valve strongly arcuate in the upper part. Dorsal valve 

 unknown. 



Surface of the ventral valve covered by very fine, even, rounded, 

 thread-like striae, seven to eight of which may be counted in the space of 

 an eighth of an inch on the front of the shell, but are much finer near the 

 beak. The striae have been marked by numerous fine, slender spines, the 

 bases of which can be seen protruding through a portion of shale adhering 

 to the surface of one of the specimens; but they are so fine as to make 

 scarcely any perceptible scar on the surface of the striae when denuded. 



The species is of the general form of many of those usually referred to 

 P. Prattenanus Norwood, but differs materially from the original specimen 

 used and figured by Dr. Norwood in the finer striae and short hinge-line. 

 It does not appear to be positively identical with any of the forms figured 

 by De Koninck as P. Cora, but is a very closely representative species.* 



Formation and locality. — In limestone in the higher parts of the Lower 

 Carboniferous, north of Snowstorm Hill, Dry Canon, Oquirrh Mountains, 

 Utah. Collected by J. E. Clayton. 



Pkoduc'I'US sbmiketiculatus Martin. 



Plate 5, jBigs. 5-6. 



The specimens of this species in the collection have very much the 

 form and characters of those of the species which occur in the Burlington 

 and Keokuk beds in Iowa and Illinois, represented by the specimen figured 

 in the Geological Report of Iowa (vol. i, part 2, plate 19, fig. 4), except that 

 they are only about two-thirds as large as that individual. The shell is 

 rather narrow and strongly arcuate; the beak narrow and rather pointed, 

 and distinctly separated from the body of the shell; the hinge-line appears 

 to have been about equal in length to the width of the shell below; the 

 sides of the sliell abrupt, flattened, and squarish, while the middle of the 



* The original specimeu of P. Icevicostiis is from the base of the Lower Carbon- 

 iferous, and from the beds at Burlington, Iowa, referred to the Waverly group; but 

 there aie forms of very similar character iu other beds of the formations at other places 

 that cannot be readily distinguished from those above referred to. 



