MUSEUM OF COMPAEATIVE ZOOLOGY. 273 



darkly clouded on the upper posterior part, and is divided here by a pale ray 

 from the umbones to the lower posterior margin. The sparsely set micro- 

 scopic tubercles can only be observed with a magnifier; to the eye the surface 

 looks shining and smooth, not unlike that of Kellia suborbicularis. Taking it 

 for granted that the specimens labelled by Dr. Jeffreys are really the same as 

 his type in the British Museum, and authentic, it follows that Lyonsidla or 

 Pecchiolia subquadrata Jeffreys is a congener of the Callocardiae of Smith, 

 though, as Smith himself indicates, not of the original Callocardia. They 

 have a hinge much nearer the original Isocardia than Kelliella has, but differ- 

 ent from either ; and, if they were two inches in diameter, would be unani- 

 mously accorded separate names. As the element of size can hardly, on 

 reflection, be considered in systematic work as opposed to definite characters, 

 I have taken the responsibility of separating them, leaving to my more 

 "conservative" friends the usual option of disregarding the distinctions if 

 they prefer. 



Through the extreme courtesy of Mr. Smith, I have received from him an 

 enlarged drawing of the type of Callocardia showing the hinge. It is impos- 

 sible without a figure to exhibit clearly the marked differences which exist 

 between the hinges of Callocardia and Vesicovvja. This I hope, later, to 

 supply. 



The species then comprise Callocardia (^Vesicomya) subquadrata (Jeffr.) Dall, 

 and C. (F.) atlantica, C. {V.) pacijlca, and C. {V.) Adamsi Smith; C. (F.) 

 pilula and venusta Dall. 



Lest there should be a question as to the species, I retain the description 

 I had prepared before the publication of the Challenger report, and which I 

 should have otherwise (as in other cases) suppressed in this paper. 



Callocardia (Vesicomya) atlantica Smith. 

 Callocardia (?) atlantica Smith, Chall. Rep. Lam., p. 157, pi. vi. fig. 8, 1885. 



Shell small, extremely fragile, rotund, polished, whitish, ^^dth a pale filmy 

 epidermis ; sculptured evenly all over with fine close-set concentric rounded 

 lines or threads; the margin of the valves is nearly circular, above which rise 

 the small prominent, inflated beaks; they are nearly median as regards the 

 ends of the valves, but extend a little forward of the median line; both ends 

 and the base are subequaUy rounded ; a ridge extends (as in Cetoconclid) 

 from the beaks backward, including between itself and the hinge margin a 

 narrow inbent area; the ligament is wholly external, long linear, black; there 

 is no internal cartilage ; in each valve is a thin short rectangular lamellar 

 cardinal tooth parallel with the hinge-line and immediately contiguous to a 

 lamellar elongated tooth, between which and the margin is a sharply cut 

 groove extending far behind the tooth ; interior polished, white ; muscular 

 Bears invisible. Lon. of shell 4.5, alt. 4.5, diameter about 4.0 mm. 



VOL. XII. — NO. 6. 18 



