PECTUNCULUS. NUCULA. 69 



orifices, but under the posterior termination of the branchiae 

 there is a very short anal duct issuing from the body, with a 

 small round reflexed margin. 



The foot in the live animal is snow-white, without a byssal 

 gland and groove, but deeply cloven in 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 pointed termination ; when the foot is not fully 

 protruded, it forms three or four folds at the margins; 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 branchise of the 

 same size, which, instead of having the transverse striae or 

 circulatory vessels linear, present the appearance of white wavy 

 ringlets, and are crossed by other sinuous lines ; both surfaces 

 have much the same depth of markings. The branchial laminae 

 hang very obliquely, and from the smaller or anterior extre- 

 mity a pah 1 of very narrow, light brown, linear labia or palpi, 

 smooth on both sides, originate, 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 linear connecting branchial palpi greatly resemble each 

 other. 



NUCULA, Lamarck. 

 N. NUCLEUS, Linnaeus. 



IV. nucleus, Brit. Moll. ii. p. 215, pi. 47- f. 7, 8 ; (animal) pi. P. f. 4. 



N. decussata, Brit. Moll. ii. p. 221, pi. 47- f. 1, 2, 3. 



N. radiata, Brit. Moll. ii. p. 220, pi. 47. f. 4, 5, and pi. 48. f. 7- 



Animal suboval, thick, having the ventral portion of the 

 body mottled flake-white ; the liver is dark green, with the 



