604 Contributions to Invertebrate Palaeontology. 



at Webb Summit, retaining all the fimbriae of the surface in a most 

 perfect manner, an enlarged figure of which is given on Plate XVI, 

 fiff. 8. 



'O* 



Genus ATHYRIS McCoy. 



Atbyris siibtilita. 



Plate XVI, figs, 7-9. 



Terehmtula suhtUita Hall, Stansbury's Kept. Great Salt Lake, 1852, p. 409, 

 pi. iv (by error in text pi. ii), fig. 1 a, b, and 2 a, b. Terehratula subtilita, 

 Athyris subtilita, and Spirigera subtilita of various authors. 



Internal casts of specimens of this species, of small size, are 

 common in the upper chert beds of the Coal Measures in Hocking 

 County, Ohio. Individuals have been observed varying in size 

 from less than one-eighth of an inch to more than half an inch in 

 diameter, but all in the condition of casts. The larger specimens, 

 although much smaller than those usually found in the shaly lime- 

 stones at Greentown and elsewhere in Ohio, nevertheless show dis- 

 tinctly by their markings and the distinctness of the muscular scars 

 that they were adult shells, but probably stunted in growth by 

 unfavorable conditions, as they are perfect in form and markings. 

 The specimen illustrated on Plate XYI, figs. T-9, is from the cherty 

 layers at Mrs. Banks, in the railroad cutting. Falls Township. The 

 individuals from the black cherts, at Webb Summit, Hocking 

 County, are of larger size, and correspond more nearly with the 

 ordinary forms of the species. 



MOLLUSCA. 

 LAMELLIBRANCHIATA. 



Genus AVICrLOPECTEN McCoy. 



ATiciilopecteii iiiterliiieatiis. 



Plate XVI, figs. 10 and 11. 



Avicidoppxten interlineatus M. and W., Proc. Acad. Nat. Sci. Phila., 1860, p. 454; 

 Geol. Kept. 111., vol. 3, p. 229, pi. 2G, fig. 7. 



Shell small, subcircular in outline, hinge-line straight, nearly as long as 

 the width of the shell below, and with sharjjly angular auriculations. Valves 

 very gently convex. Surface of the left valves marked by fine, even, lamel- 

 lose, concentric striae ; several of which are more strongly and highly elevated, 



