Ort}us (^Platystrophia) crassa — Mel: 



21 



117, plate 10, fig. 3, :\Ir. Meek describes a s-hell a.s OdJus dentata. 

 Pander, and expressess doubts as to its being the form intended liy 

 Pander for that species. The specimens were wrongly put up for jNIr. 

 Meek. Pander's 0. dendata not being sent. 



I now propose the above name for the shell described by Mr. ]\ieek. 

 After comparing with 0. latkodata, James, he says : 



" Its hinge line, generally, about equals the greatest breadth of the 

 valves, but may be a little less, or a little greater, than the breadth. 

 Its lateral slopes are always abrupt, and much less compressed than 0. 

 laticostata; but, like that variety, its mesial sinus is large, and very 

 profound, with angular margins, and its mesial fold strongly elevated. 

 In old individuals the valves become often remarkably gibbous, the 

 convexity exceeding the length and nearly equaling the breadth, 

 being much increased by the prominence of the mesial fold, and the 

 elevation of the plications forming the margins of the sinus. It differs 

 from 0. laticodata in generally having but a single strong plication m 

 the sinus, and only two on the mesial fold ; though we sometimes see 

 an individual with the commencement of a second smaller plication oc- 

 cupying one of the slopes of the sinus on one side of the middle one, 

 and very rai;ely one with a third one on the other side. Where rudi- 

 mentary plications exist in the sinus, they usually give origin to cor- 

 responding, partly developed plications on the slopes of the mesial fold 

 of the other valve. All the plications are simple, and excepting the 

 rudimeutai-y ones mentioned, quite coarse, prominent and angular ; 

 the number on each lateral slope being constantly five or six." 



I will add, that different individuals of this species vary considerably, 

 some of the smaller ones being less than half an inch long and five- 

 eighths inch wide; the larger ones over an inch long and one and 

 a quarter inch wide. Some specimeus, the cardinal line is not over 

 half the greatest width of the shell ; others, it is more than the great- 

 est width below ; but, taking a number of individuals, the difference 

 from one to the other is found to be very gradual. 



Position atid locality : Cincinnati Group, between 300 and 400 feet 

 above low water-mark of the Ohio River, at Cincinnati. 



OrtJiis (f) Morroivensis.— (J A^iES.) 

 Shell oval, broader than long, cardinal area narrow, and about or.e 

 third of the greatest width of the shell ; foramen narrow, extending tj 

 the beak ; dorsal valve convex, sloping rather abruptly from a mesial 

 elevation to 'the free margins ; mesial elevation extending to the 

 front; beak incurved; ventral valve convex, with a triangular 

 mesial sinus, narrow at the beak and spreading out rapidly to the 



