NATURAL SCIENCES OF PHILADELPHIA. * 325 



shorter anterior, and straighter basal outline, to be referred to 

 that species, even if found in rocks of the same age, while the 

 rather wide interval between the horizons at which the two forms 

 occur, renders it still more improbable that the}^ belong to the 

 same species. 



It is extremely difficult to arrive at correct conclusions in re. 

 gard to the generic affinities of such shells, from the stud}^ of 

 mere casts, and it is, therefore, only provisional!}^ that I have 

 referred this and the last-described species to the genus Sedg- 

 ivickia. We may rest quite well assured, however, that paloeozoic 

 forms of the kind cannot be properly referred to the existing 

 genus Anatina. 



Locality and position Same as last. Mr. James's collection. 



SEDGWICKIA (GRAMMYSIA?) NEGLECTA, Meek. 



Shell transversely ovate, about one-third longer than high, 

 rather distinctly compressed, most convex and most elevated in 

 the central and umbonal regions, and compressed-cuneate behind ; 

 anterior margin rounding from the lower end of the lunule into 

 the base, which forms a nearly semi-oval curve, its most promi- 

 nent part being near the middle ; cardinal margin apparently 

 straight, and declining posteriorly from the beaks ; posterior 

 margin rather narrowly rounded ; beaks moderately prominent, 

 and scarcely one-third the length of the valves from the anterior 

 margin. Surface ornamented with regular, distinct, but not very 

 prominent concentric costffi, tliat become suddenh' obsolete on 

 the posterior third of the valves. Lunule narrow, but sharply 

 defined. 



Length, about 1.04 inch; height, 0.67 inch; convexity, about 

 0.35 inch. 



The only specimen of this species I have seen is an external 

 cast of the right valve, which has evidently been, to some extent, 

 accidentally compressed in the region of the beak, but the shell was 

 certainly never ver}'^ convex. It has almost exactly the general 

 aspect and kind of ornamentation seen in the t3'pical forms of 

 Sedgivickia, a group which, since it was first proposed bj' Prof. 

 McCoy, has been included bj^ him in his genus Leptodomus, which 

 seems to me to have been originally founded on a very distinct 

 type. 



Our shell also resembles. rather closely some forms apparently 



1872.] PART TIL— 22 



