84 PALEONTOLOGY OF KENTUCKY. 



examples, slightly curved in adult specimens, anterior margins rounded to the 

 front. 



Dorsal valve greatly convex, particularly towards the front, approaching 

 which the mesial ridge is greatly elevated (especially in more globose speci- 

 mens), and is gathered into four plications. The sides radiate from beak in 

 five or six plications, strongly curved towards the anterior lateral margins. 



Ventral valve is depressed from beak to anterior central region, where it 

 curves more strongly. A mesial sinus begins at beak, has sloping sides, three 

 plications, and diverges to a great width at the front, where it terminates in an 

 extended, half -circular, marginal projection corresponding to, elevation in the 

 front of the dorsal valve. The sides radiate to the anterior margins in five or 

 six plications rounding towards the front. 



The beak of the ventral valve is curved so as to hide the apex of the dorsal 

 valve. The angular plications and furrows give the margins a distinct zigzag 

 outline; the larger specimens show several lines of growth, more distinct 

 towards the front, crossing the plications and furrows. 



In the specimens before me the plications in the lateral slopes vary ; some are 

 imperfectly developed and almost indistinct. The plications in the mesial sinus 

 and elevation never vary. The smaller specimens are generally more com- 

 pressed and the larger more rounded in outline than the typical forms. 



Formation and Locality. This shell seems to be confined to the Upper Trenton, where it is found 

 quite abundantly, and associated with Orthis borealis. It occurs in most of the blue-grass counties of the 

 State. The specimens figured are from Frankfort, Ky. 



Genus Rhynchotreta. 



Rhynchotreta, Hall. 28th Regent's Report 1879. 

 Etymology : rhynchos, beak ; tretos, with a hole in it. 



Type : Rhynchonella cuneata, Dalman. 



Shell triangular ; surface with angular plications. Ventral beak straight, 

 produced beyond the dorsal beak, extremely perforated; foramen with an 

 elevated margin ; space between the foramen and hinge-line occupied by a del- 

 tidium in two pieces, being divided by a longitudinal suture, and transversely 

 striated. Valves articulated by two slender, curving teeth, proceeding from a 

 broad curving hinge-plate in the ventral valve, which fit into corresponding 

 sockets in the dorsal valve. Crurae rising from near the dorsal beak, and curv- 

 ing into the ventral cavity, and thence recurved towards the dorsal side, and 

 probably uniting. Structure fibrous, and apparently yery minutely punctate. 



