CARDIUM. 103 
smaller and of less depth than the under one; they hang more 
vertically than those of its congeners, and the transverse 
pectinations are more conspicuous without than within ; the 
palpi are very long, triangular, united by a labium around the 
mouth, and with the branchial lamimze by a membranous 
filament; they are more strongly striated on the inner than 
on the outer areas. 
This elegant species is frequently obtained by the dredge in 
the coralline district at Exmouth. 
The following species have not occurred to us alive :— 
C. rusticum, Linnzeus. 
C. rusticum, Brit. Moll. ii. p. 11, pl. 31. f. 3, 4: 
C. acuteatum, Linnzus. 
C. aculeatum, Brit. Moll. ii. p. 4, pl. 33. f. 1. 
C. pyem&uM, Donovan. 
C. pygmeum, Brit. Moll. ii. p. 29, pl. 32. f. 8; (anmal) pl. N. f. 2. 
C. suEecicum, Reeve. 
C. suecicum, Brit. Moll. ii. p. 33, pl. 32. f. 6. 
The last may be a young delicate var. of C. edule. 
MACTRID ZA. 
This is one of the old Linnzan families, with only a single 
genus, so distinctly characterized as to require few remarks ; 
it contains four British species. By its open mantle and short 
tubes it is allied to the Veneres, but we cannot concur in the 
opinion of some authors, that it presents many features of the 
Myade. We take the liberty of saying, that it will be very 
difficult to find a single accordant poimt between the two 
familes, except the generalities of every bivalve. Mactra 
and Mya are far asunder: the former has the mantle open, 
short tubes, and very circumscribed in length, suboval, broad 
branchiz ; in the latter, the branchiz are narrow, elongated, 
with a part of them lyig in the branchial tube ; the mantle 
is quite closed, and the siphonal apparatus particularly long. 
The dentition of the two families is entirely different, with the 
