MOLLUSCA. 613 



1905. Legumen appressum Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 17. 

 1905. Legumen ellipticum Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 17. 



Description. — The dimensions of two specimens are : length, 

 74 mm. and 34 mm. ; heig'ht, 35 mm. and 15 mm. Shell subellip- 

 tical in outline, the beaks small, appressed, scarcely projecting 

 above the hinge-line, situated about one-fourth the length of the 

 shell from the anterior extremity. Hinge-line slightly arcuate; 

 anterior margin more or less sharply rounded, the greatest exten- 

 sion at the mid-height of the shell ; basal margin gently convex ; 

 posterior margin a little more broadly rounded than the anterior, 

 the greatest extension usually a little above the middle. Valves 

 depressed convex, without an umbonal ridge ; the surface curving 

 a little more abruptly to the cardinal margin. In internal casts 

 the anterior muscular impression is usually well defined and is 

 bounded posteriorly by a shallow furrow-like depression which 

 curves forward below; the posterior muscular impression incon- 

 spicuous. Surface of the shell marked by inore or less regular, 

 concentric lines of growth which become stronger upon the pos- 

 terior slope. These markings are usually^ impressed upon the 

 surface of the internal casts. 



Remarks. — Three species of this genus have been recognized 

 in the Cretaceous faunas of New Jersey and the South, and two of 

 these have been recognized by Whitfield in New Jersey. These 

 three species, all of them described by Conrad, have been based 

 upon very slight differences in the details of outline, and a 

 careful study of numerous examples in the recent collections of 

 the Sui-vey, from several different hoirizons, besides the speci- 

 mens in the collections at Washington and Philadelphia, has led 

 to the conclusion that all of these forms represent a single some- 

 what variable species. No two^ specimens examined agree exactly 

 in the outline of the shell- and there seem to be intermediate 

 variations between all the different species which have been 

 described. The differences in the surface markings is doubtless 

 due to different degrees of erosion. Gabb arrived at this same 

 conclusion regarding the three species in 1876. The hinge of 



