217 



somewhat beyond the hinge line: notch shallow, the grooves on the 

 posterior faces of the apopnyses very faint. 



Ratio of breadth to length in an average adnlt specimen about as 11 

 to 8. 



This species can not be referred to the 0. (Tcrehratulitcs) umhraciilum 

 of Schlotheim, from which it differs in the less length of the hinge line, 

 fewer number of plications, greater proportionate height of the area, 

 Avhich in the present species tends to become strongly retrorse in mature 

 individuals, and the subquadrate rather than semicircular outline of the 

 j^hells. The figures of Schlotheim's species also show it to possess a 

 strongly quadrilobate cardinal process, Avhile in the present form the 

 notch in the process is very shallow and the grooves very faint. 



The species to some extent resembles 0. lais, from which it differs 

 in the form of the cardinal process and in the greater proportionate length 

 of the latter. 



Development. — In the search for specimens of this rather rare species 

 (about 50 specimens were found among several thousand of the common 

 Spergen hill forms) a number of very young stages was obtained. While 

 even the adult individuals share in the general stunting so characteristic 

 of the entire Spergen hill fauna, no complete specimen in the writer's 

 <-ollection having a length of more than 5mm,* nevertheless these larger 

 individuals present the usual features of maturity. 



The smallest individual observed has a breadth of .9mm and a length 

 of .Gmm. In this specimen the ventral valve is roughly conical, though 

 slightly more convex toward the beak, which projects over the hinge line 

 and is very, prominent. The surface shoAvs IS plications as against 40 in 

 the largest individual observed, while the posterior third of the shell is 

 Avithout surface ornamentation except a few obscure concentric markings. 

 The area is high and the deltidium less sharply marked off from it than 

 in the older specimens. The dorsal valve has its greatest convexity at the 

 center and is also smooth for a considerable distance from the beak. 

 It shows no sign of an area. 



Individuals of the length of 2mm have the area perpendicular to the 

 plane of separation of the valves, and the ventral valve showing a slight 

 concavity toward the front. The number of plications also has increased 



•"'Since the above was written, the author has found at Stinesville, Monr.e County, In- 

 <liana, specimens of this species over one inch in breadth, hut agreeing in all essential 

 featur.s with the adult specimens described here. 



15— A. OF Science. 



