PECTUNCULUS. NUCULA. 69 
orifices, but under the posterior termimation of the branchize 
there is a very short anal duct issuing from the body, with a 
small round reilexed margin. 
The foot in the live animal is snow-white, without a byssal 
gland and groove, but deeply cloven im the longitudinal line 
of its base to effect a subdiscoidal extension ; it springs from 
the centre of the basal portion of the body; when in full 
action it ranges anteally and posteally, forming an elongated 
suboval disk at the central portion, and tapers at each end to 
a moderately pomted termination ; when the foot is not fully 
protruded, it forms three or four folds at the margims; the 
animal does not execute a direct progressive locomotion, but 
only turns the shell round on its disk, or from side to side. 
There are on each side a pair of symmetrical branchie of the 
same size, which, instead of having the transverse striz or 
circulatory vessels linear, present the appearance of white wavy 
ringlets, and are crossed by other smmuous lines; both surfaces 
have much the same depth of markings. The branchial laminz 
hang very obliquely, and from the smaller or anterior extre- 
mity a pair of very narrow, ght brown, Imear labia or palpi, 
smooth on both sides, origmate, and pursue their course 
around the mouth, and meet with the fellow pair. In their 
linear form and drooping position, these appendages much 
resemble the labia or palpi of Nucula. 
This species is frequently taken alive in the dredge in the 
coralline zone at Exmouth. We observe a close alliance be- 
tween Pectunculus and Nucula; both have the same sort of 
suboval foot disk, and a similar quality of progression ; and 
the lmear connecting branchial palpi greatly resemble each 
other. 
NUCULA, Lamarck. 
N. nucievs, Linneus. 
N. nucleus, Brit. Moll. it. p. 215, pl. 47. f. 7, 8; (animal) pl. P. f. 4. 
N. decussata, Brit. Moll. ii. p. 221, pl. 47. f. 1, 2, 3. 
N. radiata, Brit. Moll. ii. p. 220, pl. 47. f. 4, 5, and pl. 48. f. 7. 
Animal suboval, thick, having the ventral portion of the 
body mottled flake-white; the liver is dark green, with the 
