470 GEOLOGY OF OHIO. 



is scarcely a chance of mistaking them. They are also usually of about 

 the same size with the western specimens, only occasionally an individual 

 occurring of large size; in which case they present the characters of the 

 . form described by Mr. McChesney as P. fasciculatus. 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 striae and apparent absence of spines, are the re- 

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

 other origin. 



Productus pileilorviis. 



Plate IX, figs. 13 and 14. 



Productus pileiformis McChesney, New Pal. Foss., 1859, p. 40. 

 Com. Productus cora D'Orb. 



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

 from side to side, beak small, somewhat pointed, and the body of the shell some- 

 what gradually expanding toward the front. Hinge-line usually quite short and in- 

 conspicuous; 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 ventral valve, the added ones at firs£ very 

 small, presenting a strongly alternating character, soon becoming of full size. The 

 strias of the dorsal valve do not present this feature on any of the specimens ex- 

 amined. Body of the shell marked in the upper part by numerous strong, irregular 

 and unequal, transverse 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 are, however, many 

 points of difference 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 strise. There is perhaps equal 

 reason for uniting with P. cora, P. tenuicosta Hall, P. coriformis Swallow, 

 and some others ; but these names seem useful and convenient in desig- 

 nating forms of different horizons and geographical areas; as there are 

 differences between them readily recognized and appreciated by those ac- 

 customed to examining them, which cannot be portrayed in a figure or 

 described verbally, but which often serve to detect, or at least aid in de- 

 tecting the true horizon of beds of rock which would otherwise be left 

 in doubt, and it appears necessary to have some means of referring to or 

 designating such forms when speaking of beds characterized by them. 

 The form under consideration resembles those from the Chester lime- 

 stone of Illinois, used in the description given by Mr. McChesney, more 

 closely than they do those from the St. Louis limestone, given under the 

 name P. tenuicosta by Prof. Hall, which have a larger and more rounded 

 beak and much longer hinge-line; the striae, however, in their extreme fine- 

 ness resembles those of the St. Louis limestone specimens. 



