CAKDIUM. 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 laminae 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, Linnaeus. 



C. rusticum, Brit. Moll. ii. p. 11, pi. 31. f. 3, 4. 

 C. ACULEATUM, Linnaeus. 



C. aculeatum, Brit. Moll. ii. p. 4, pi. 33. f. 1. 

 C. PYGM.EUM, Donovan. 



C. pygmteum, Brit. Moll. ii. p. 29, pi. 32. f. 8 ; (animal) pi. N. f. 2. 



C. SUECICUM, Reeve. 

 C. suecicum, Brit. Moll. ii. p. 33, pi. 32. f. 6. 



The last may be a young delicate var. of C. edule. 



MACTRID.E. 



This is one of the old Linnaean 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 

 Myadas. We take the liberty of saying, that it will be very 

 difficult to find a single accordant point between the two 

 families, except the generalities of every bivalve. Mactra 

 and My a are far asunder : the former has the mantle open, 

 short tubes, and very circumscribed in length, suboval, broad 

 branchiae ; in the latter, the branchiae are narrow, elongated, 

 with a part of them lying 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 



