NATURAL SCIENCES OF PHILADELPHIA. 17 



on its superior border ; the other pit smaller, deepest on its superior border 

 Surface with fine, indistinct strife of growth. 



Length -32 (100) ; height -16 (50) ; length of short end -13 (41) ; of long 

 end -19 (59) ; thickness of both valves '13 (41). 



ISOCARDIA Lamarck. 

 Isocardia ? Jesn^, n. sp. Shell of rather large size, triangular, very 

 ventricose. Beak elevated above the hinge, flattened, incurved and directed 

 forward, with a deep lunule in front. Hinge-line extended posteriorly ; alate 

 expansion more or less flattened, posterior border truncated at right angles 

 with the dorsal line, elongate ; antero-ventral margin nearly straight through- 

 out the lower two-thirds of its length, forming an angle of 50 with the 

 posterior border, above gradually curved to the lunette. An elevated sharp 

 umbonal ridge runs sigmoidally from the beak, arching first backwards, then 

 forwards, and again backwards to the posterior ventral angle ; the posterior 

 slope from this ridge becomes more and more steep in approaching the beak, 

 at and near which it faces dorsally, and is overhung by the umbonal ridge. 

 A shallow sinus appears in the middle of the antero-ventral margin, from 

 which a furrow ascends toward the beak. Surface marked with faint incre- 

 mental lines which, on the antero-ventral slope near the base, become dis- 

 tinct, and nearer the beak rise in well marked wrinkles converging in the 

 lunette. Greatest length from beak along the umbonal angle 1-27; length 

 of hinge-line "81 ; length of shell posterior to the beak *65 ; length of posterior 

 side -90. 



This species, in its essential features well marked, seems, nevertheless, to 

 be quite variable. In some specimens the posterior alate portion is much 

 more developed. In others it is less developed, and the umbonal ridge is 

 more acute, giving the shell the appearance of being truncated through its 

 thickest part. 



There is no direct evidence of the affinities of this species with Isocardia, 

 except the swollen umbones and enrolled beaks ; and it is so referred, 

 mainly, in deference to high authorities who have made similar disposition of 

 such forms. (See Sandberger, Verstein., pi. xxvii. fig. 11). It has the 

 posterior wing of the Aviculidaz, and also the ventricose left valve and 

 enrolled beak of Aucella in that family. Yet it differs from Amelia in hav- 

 ing the right valve equally ventricose, and in being entirely destitute of an 

 anterior wing and byssal sinus. The sharp, sigmoid umbonal angle is a fea- 

 ture seen in some species usually referred to Cypricardia (See C. bipartita de 

 Kon. Anim. Foss., p. 94. pi. fig. 15). 



Finally, it is worthy of mention that the peculiar angular form of this species 

 is but an exaggeration of the characters of Sanguinolites cylindricus. It is 

 relatively shorter, more ventricose and more flattened on the posterior cardi 

 nal angle. If external form is to be the basis of family distinctions and alliances, 

 (See Agassiz, Cont. to Nat. Hist, of N. A.) then Isocardia Jennoz will go with 

 Sanguinolites cylindricus, Win., S. decipiens, McCoy, Cypricardia rigida,W. and 

 W., C. bipartita, de Kon., Isocardia coelata, Sandb., &c, &c, into a family 

 whose circumscription has not yet been marked out. 



CARDIOP3IS, Meek and Worthen. 



Cardiopsis megambonata, Win. (Proc. Acad. Nat. Sci., Sept., 1862, p. 417.) 

 The specimen here referred is many times larger than the types of the spe- 

 cies, being of the size of C. crenistriata, Win., from which it differs principally 

 in the coarser and more rigid ribs and more prominent beak. 



The ribs in the Burlington specimens do not increase in number with age, 

 and scarcely increase in size ; the intercostal spaces are flat, gradually 

 widening. 



Height from beak to ventral margin -91 (100); length -85 (97); thickness 

 of left valve -55 (60). 



1863.] 2 



