582 Contributions to Invertebrate Palaeontology. 



The Ohio specimens of the species are of very characteristic form, 

 so closely resembling those from the limestone at Chester, 111., that 

 there is scarcely a chance of mistaking them. They are also usuallv 

 of about the same size with the western specimens, only occasion- 

 ally an individual occurring of large size; in which case they 

 present the characters of the form described by Mr. McChesney as 

 P. fascicidatus. A few individuals have been noticed from the 

 harder and more compact limestone, which are less quadrangular in 

 the upper part, and the striae appear a little finer and smoother, and 

 the front of the shell in its extension somewhat rounder than the 

 usual form. Some of these peculiarities, especially the smooth finer 

 appearing strias and apparent absence of spines, are the result of 

 excessive exfoliation, but the difference of form is probably of other 

 origin, 



Prodtictiis pileifoiniis. 



Plate XIII, figs. 13 and 14. 



ProductHS pilei/ormis McChesney, New Pal. Foss., lSo9, p. 40. 

 Comp. Productus cora D'Orb. 



Shell of medium size, pileiform, highly arcuate from Leak to front and 

 rounded from side to side, beak small, somewhat pointed, and the body of the 

 shell somewhat gradually expanding toward the front. Hinge-line usually 

 quite short and inconspicuous ; auriculations small or obsolete. Surface of 

 the shell marked by very fine radiating striae which are even and usually 

 quite smooth or free from spine-bases ; increased by implantation on the ven- 

 tral valve, the added ones at first very small, presenting a strongly alternating 

 character, but soon becoming of full size. The stria? of the dorsal valve does 

 not present this feature on any of the specimens examined. Body of the shell 

 marked in the upper part by numerous strong, irregular and unequal, trans- 

 verse undulations, a portion of which only are projected entirely across the 

 shell on the ventral side. 



There is some question as to the propriety of separating these 

 forms from P. cora D'Orb. of the Coal Measures; there arc, how- 

 ever, many points of dilfercnce as well as many of resemblance, 

 although none of them on either side are very constant beyond 

 limited localities ; except perhaps the general form and usually fine 

 striae. There is perhaps equal reason for uniting with P. cora, 

 P. tenuicosta Hall, P. cori/ormis Swallow, and some others ; but 

 these names seem useful and convenient in designating those of 

 different horizons and geographical areas ; as there are diff'crences 

 between them readily recognized and appreciated by those accus- 



