42 CONCHOLOGY. 
thick, smooth, or without tentacular papille, augmented behind 
by two indistinct tubes ; mouth small and oval; labial appendages 
narrow ; branchial lamine very small, and united in their length 
among themselves and with those of the opposite side ; foot oval, 
trenchant, very long. 
Shell. The Mactra has a peculiar hinge distinguishing it from 
all other genera. It is triangular, with a curved or angular com- 
pressed tooth on each valve, with a small oblique cavity on each 
side, to which is attached the ligament. ‘T'wo lateral teeth, one 
near the primary tooth and one near the ligament. ‘The shape 
of the shell is subtriangular or oblong; exterior smooth, striated, 
or ribbed transversely ; two muscular impressions united by a 
narrow marginal tongue. Inhabits the British and American 
coasts. Forty-four species. 
Mactra spengleri. Mactra helvacea. 
SSSSS 
. striatella. 
. carinata. 
. straminea. 
. Australis. 
violacea. 
. fasciata. 
. turgida. 
. plicataria 
rufescens. 
. maculata. 
. subplicata. 
<zzeeeeeee 
. lactea. 
. abbreviata. 
. crassatella. 
= ss 55 
gigantea.* 
. solida.* 
. oblonga.* 
. californica.* 
. arctata.* 
. nucleus.* 
. triangularis. 
M. grandis. 
M. stultorum. 
M. maculosa. 
M. ovalina. 
M. castanea. 
M. rufa. 
M. squalida. 
M. Brasiliana. 
M. donacina. 
M. depressa. 
M. lilacea. 
M. trigonella. 
M. deitoides. 
M. alata. 
M. alba.* 
M. similis.* 
M. fragilis.* 
M. lateralis.* 
M. deaurata.* 
M. planulata.* 
M. tellinoides.* 
