88 PALAEONTOLOGY OF KENTUCKY. 



Surface marked by regularly imbricating lamellose lines of growth, which, 

 on the surface of well preserved specimens are finely crenulate on their edges 

 and the intermediate spaces striate. In most of the specimens found the sur- 

 face is entirely smooth, with the exception of a small strip around the lateral 

 and basal margins, where the lines of growth are not obliterated by the silicifi- 

 cation. 



Formation and Locality. Found in great abundance and in well preserved specimens in the Cor- 

 niferous limestone at and around the Palls of the Ohio, in Kentucky and Indiana. In some washes of the 

 fields around Charlestown the ground is, after some hard rains, literally covered with these pretty little 

 shells. This species bears in its outlines the greatest resemblance to Athyris concentrica of Europe, with 

 which it is undoubtedly nearly related, but it may be easily distinguished from that species by not having 

 the fine concentric lines which are so conspicuous in the European species. From Athyris spiriferoides it 

 differs partly in shape, and, according to Prof. Hall's statement, mostly in internal features. Specimens 

 are found showing the internal spiral coils splendidly preserved. The figures 25 and 31, on plate 16, do 

 not~show plainly enough the round foramen or perforation in the beak of the ventral valve, 



Genus Atrypa. Daiman. 



Atrypa, Daiman. Vet. Acad 1827. 



Etymology: a, without; trypa, a hole.. Daiman supposed that this shell had no foramen beneath the beak, 



which it has ; A is, therefore, a misnomer. 



Shell inpunctate and fibrous ; surface often or usually ribbed, and often fur- 

 nished with imbricating lines of growth, often produced into foliaceous ex- 

 pansions ; valves articulated by teeth and sockets ; the ventral valve often 

 depressed in front, with or without a mesial sinus ; its beak incurved and 

 perforated at the apex by a minute foramen, which is sometimes bounded in 

 front by a deltidium. Dorsal valve convex and often very ventricose, with 

 or without a mesial fold ; the hinge-plate divided and supporting two large 

 conical spires, which are directed into the hollow of the dorsal valve. 



Atrypa aspera. SCHLOTHEIM. 



Plate XI V., figures 1 to 11. 

 List of synonyms, see Hall's Pal. of New York, volume 4, page 322. 



Shell of medium size ; sub-circular or oval, moderately convex ; length and 

 width almost equal ; hinge-line shorter than width of shell ; cardinal extremi- 

 ties rounded ; lateral margins gently curved ; basal margin somewhat more 

 arcuate in some specimens, while in others the curvature is less. The central 

 part of the base is, in some shells, slightly extended, while in others this fea- 

 ture does not exist. Ventral valve depressed convex, gently and regularly 

 curving from beak to base ; lateral margins slightly elevated over adjacent 

 portion of valve ; beak moderate and incurved over umbo of opposite valve. 



