ARCA. 67 
insertion of the byssal lamina, has a shallower continuation of 
it to the point. We have only seen the foot when protruded 
in a lmguiform or subcylindrical shape, but it is probable that 
the longitudinal groove can expand and assume a spatulate 
character like that of Nucula and Pectunculus ; but the animal 
is very sensitive, and during a long examination never showed 
any kind of locomotion. ‘There are on each side a pair of 
brown branchial laminze of nearly similar size, of small depth, 
which run quite horizontally; they are slightly crenated at 
the edges, and the outer surfaces are less striated than the 
inner. There are no pendulous palpi, but the linear branchize 
are continued in a narrowed form on the buccal area, and 
meeting with their fellows, constitute expanded labia instead 
of strict palpi. - 
Without entering into much anatomical detail, we may 
observe that the organs which supply the motive power to the 
circulation have a different arrangement in those Arce which 
have the beaks widely separate; m them, there are on the 
medial line of the dorsal range two ventricles, each accompa- 
nied by an auricle, instead of, as in the ordinary arrangement, 
a single ventricle with an auricle, on each side: but this de- 
viation does not extend to all the genera of the Arcade ; it 
appears to be the result of the distance of the branchial vein, 
on each side, from the motive power. In Pectunculus, in 
which the beaks are close together, and the pomts where the 
branchial veins pour the blood into the auricles are little sepa- 
rated, the ordinary structure of the single ventricle prevails. 
This species is very rare, but it has been taken by us more 
than once alive, im the corallme zone at Exmouth, in the 
sinuosities of masses of Serpule that are deposited in old 
bivalves. 
A. LacTEa, Linnzeus. 
A. lactea, Brit. Moll. u. p. 238, pl. 46. f. 1, 2, 3. 
Animal oblong, thick, body white; mantle pale red towards 
the middle of the dorsal range, entirely open, having the under 
surface of the ventral line marked with irregular flake-brown 
blotches on a pale yellow ground, and the upper, for some 
F2 
