ARCA. 67 



insertion of the byssal lamina, lias a shallower continuation of 

 it to the point. We have only seen the foot when protruded 

 in a linguiform 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 laminae 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 branchiae 

 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 Area which 

 have the beaks widely separate; in 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 Arcadae ; 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 points 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, in the coralline zone at Exmouth, in the 

 sinuosities of masses of Serpulte that are deposited in old 

 bivalves. 



A. LACTEA, Linnaeus. 

 A. lactea, Brit. Moll. ii. p. 238, pi. 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 



