418 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



is larger, less inflated, tlie anterior end higher, the base more rounded, the pos- 

 terior end more angular and proportionally longer. 



The preceding species are all of a generally suboval form with convexly arcuate 

 base, and with the hinge teeth compressed dorsoventrally, so as to seem almost 

 parallel with the hinge plate. A single enormous form, known as yet only by 

 dead valves, differs so much from the others, not only in size but general aspect, 

 while retaining essentially all the fundamental characters of the genus, that it 

 seems best to jjrive it a sectional name to itself. 



ARCHIVESICA Ball, nov. 



Shell inflatedly modioliform, mesially slightly constricted, with the hinge plate 

 short and broad and the hinge teeth radially disposed ; lunule not circumscribed 

 by an impressed line ; pallial Hue without a sinus, but descending nearly vertically 

 from the middle of the posterior adductor scar. Type, Vesicomya gigas Ball, 

 Gulf of California. 



Vesicomya (archivesica) gigas Dall. 



Plat* 16, figure 9. 



Callocardia gigas Dall, Proc. U. S. Nat. Mus., 1896, 18, no. 1034, p. 18. 



Shell large, rather thin, inflated, with a thin, wrinkled, olivaceous epidermis 

 over an earthy, concentrically, irregularly striated surface ; beaks low, inconspicu- 

 ous ; lunule and escutcheon somewhat impressed, but not limited by any distinct 

 Hue ; valves elongated, recalling the shape of Modiola capax Conrad, in a general 

 way ; the anterior side shorter and less high, the base impressed in the middle, 

 more expanded in front and behind ; dorsal margin rather evenly arched ; both 

 ends rounded; internally dentition strong, like that of C. lepta, but more dis- 

 tinctly developed; ligament short (about 20 mm.), set in a groove ; interior of 

 valve somewhat radially striate; posterior adductor scar somewhat larger, the 

 pallial line set in below it, somewhat irregular but not forming a distinct angu- 

 lar sinus; margins of valve thin, smooth. Height, 63; length, 110; diam. 

 50 mm. ; vertical of the beaks, 24 mm. behind the anterior end of the shell. 



U. S. S. " Albatross," station 3009, off Concepcion Bay, in the Gulf of Califor- 

 nia, in 857 fathoms, mud, temperature 3S° F. U. S. N. Mus. 110,557. 



This relatively enormous shell was obtained only as a number of fresh valves 

 without the soft parts, but from the shell characters it can hardly be anything but 

 a giant Vesicomya. 



/■ 

 Calloj^onia Daix. 



Callogonia Dall, Bnll. Mus. Conip. Zo,,l., 188!), 18, p. 440; type, C. Iffana Dall, 

 /. c, from off Tobago, West Indies, in 880 fathoms. Proc. U. S. Nat. Mus., 

 1890, 18, p. 19. 



