532 MAZATLAN BIVALVES 
minente ; nitente, sed striulis tenuissimis, et concentricis, et 
radiantibus ; valvd alterd dent. card. post. elongato, ant. evanes- 
cente, lat. nullis ; altera 
eeeses 
One fresh, minute, and nearly perfect valve was found, which 
is described because of its abnormal dentition. The texture, 
as seen in the microscope, closely resembles chalcedony. The 
nuclear umbo is whitish. Zong. °02, lat. ‘028, alt. °006. 
Hab.—Mazatlan ; 1 valve off frond of Murex nigritus ; L’pool 
Col. 
Tablet 2529 contains the specimen. 
[Famity ARCADZ.] 
689. Arca ————-, sp. ind. (b.) 
Tablet 2524 contains a minute valve, ‘048 by °03, swollen, 
with about 20 very fine rounded nodulous ribs, with the broad 
interstices strongly decussated. 
Hab.—Mazatlan: 1 valve on Spondylus; Z’pool Col. 
[Fanny PECTINIDE.] 
690. PectEn ————, sp. ind. (a.) 
Tablet 2525 contains 2 minute valves, broad with large ears, 
whitish, spotted with olive brown. The youngest, °05 across, 
is capped at the apex with a Lima-shaped body, and this again 
with the minute oval nucleus, about ‘004 across. The shape 
and sculpture resemble Margaritiphora, the ribs being scarcely 
developed, and the cellular layer conspicuous. 
Hab.— Mazatlan ; 4 young fresh valves, off Spondylus ; 
T’pool Col. 
691. PectrEn ————, sp. ind. (b.) 
Tablet 2526 contains a fragment of a pink shell, apparently 
shaped like P. varius. 
