68 ARCADA. 
little depth, with a sand-like rusty-brown belt, and a darker 
interrupted line nearer the margin, which is finely dentated ; 
but there are no equidistant points or ocelli, as in the last 
species. The mantle is without stphonal folds. The foot is 
white with a deep fissure at the bend, and is provided with a 
green filamentous membrane for attachment, which has all the 
same incidents as in Arca tetragona. The foot is pure white, 
and can be exserted to a considerable length; it is fleshy, 
tapers cylindrically, and is very lke that of Galeomma. 
There are on each side a pair of similar-sized, very thin, pale 
yellow symmetrical branchize, which gradually taper and unite 
around the mouth, forming a double lamina, of the same 
character as in Arca tetragona and Peciunculus glycimeris. 
This species is frequently taken free in the dredge, in the 
coralline zone at Exmouth, and also fixed by the byssus 
amongst the masses of Serpule in old bivalves. 
We had in our possession the identical Arca barbata first 
introduced as British by Dr. Turton im his ‘ Conchyha Di- 
thyra, and, on showing it to Dr. Goodall, he expressed an 
opinion that it was an exotic specimen. 
We have not seen alive the 
A. RARIDENTATA, Searles Wood. 
A. raridentata, Brit. Moll. ii. p. 241, pl. 45. f. 8. 
PECTUNCULUS, Lamarck. 
P. eitycimeris, Linneeus et Auct. 
P. glycimeris, Brit. Moll. i. p. 245, pl. 46. f. 4,5, 6,7; and (animal) 
pl.) P::£. 6. 
Animal suborbicular, lentiform, bluish white; mantle open 
throughout the periphery of the shell, except the dorsal line ; 
the margin is plain, having its upper surface for the depth of 
an eighth of an inch powdered with minute sand-like, pale 
red, brown or black pots. There are at the posterior side, 
for half its length, at the verge of the mantle, about twenty- 
five minute equidistant black dots or ocelli, besides short, 
brown, transverse bars; the under surface is marked with 
short flake-white cross-lines. The mantle has neither tubes nor 
