FOSSILM OF THE LOWER CAKBONIFEROUS. 267 



the front of the shell, whicli is liroadly 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 up})er part. Dorsal valve 

 unknown. 



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

 thread-like stria?, 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 stri:c have been marked by numerous fine, slender spines, the 

 bases of which can be seen prolxuding 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. Prattcnaniis Norwood, but differs materially from the original specimen 

 used and figured by Dr. Norwood in the finer strire 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 ami localifif. — In limestone in the higher parts of the Lower 

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

 Utah. Collected by J. E. Clayton. 



PltODUCrUS SEMIRETICULATUS MlirtlU. 

 Plate 5, fiK.s. 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 Pxeport 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 nari-ow 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 shell abrupt, flattened, and squarish, while the middle of the 



* The origiual specimeu of P. Icevicontm is from the base of the Lower Carbon- 

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

 theie aic forms of very similar character iu other bedsof the formations at other phices 

 that cannot be readily distinguished from those above referred to. 



