418 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
is larger, less inflated, the 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 give it a sectional name to itself. 
ARCHIVESICA Datt, 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 line without a sinus, but descending nearly vertically 
from the middle of the posterior adductor scar. Type, Vesicomya gigas Dall, 
Gulf of California. : 
Vesicomya (archivesica) gigas Datu. 
Piate 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 
line ; 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 38° 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. 
Callogomia Datt. 
Callogonia Dall, Bull. Mus. Comp. Zodél., 1889, 18, p. 440; type, C. leeana Dall, 
l. c., from eff Tobago, West Indies, in 880 fathoms. Proc. U.S. Nat. Mus., 
1896, 18, p. 19. 
