CKENELLA. 63 



filamentary mass issues, and fixes the animal to Ascidue and 

 marine substances. The anterior part of the foot is white, 

 narrow, finger-shaped, and moderately pointed ; when in full 

 extension it takes the form of a narrow, flat tape, marked with 

 a slight brown line running from base to point; it is pro- 

 truded close to the anterior side of the byssus, but as an organ 

 of locomotion it only comes into action when the animal is 

 detached from its mooring, which it has the power of effecting 

 by withdrawing the end of the byssal lamina from the groove 

 in the heel, and it can refix itself by spinning a new byssus ; 

 this operation we have frequently seen; when fixed, the 

 foot appears to be an organ of tact, as it is often exserted, and 

 the point kept in movement as if searching or feeling. There 

 are a pair of branchial laminae on each side of the same size, 

 and smooth on all surfaces ; the palpi are long, subtriangular, 

 pale brown, and pectinated. The animal differs from Mytilus 

 and Modiola in the perfect symmetry of the four branchial 

 plates. 



This species is often attached to old bivalves and masses of 

 Serpuke, but is more usually imbedded in the coriaceous 

 mantle of the Ascidia mentula, from which twenty of all sizes 

 have been extracted. It inhabits plentifully the coralline 

 districts at Exmouth. 



We have not seen alive the following species : 



C. NIGRA, Gray. 

 C. nigra, Brit. Moll. ii. p. 202, pi. 44. f. 5, and (animal) pi. Q. f. 7. 



C. DECUSSATA, Montagu. 

 C. decussata, Brit. Moll. ii. p. 210, pi. 45. f. 2. 



C. RHOMBEA, Berkeley. 

 C. rhombea, Brit. Moll. ii. p. 208, pi. 45. f. 3. 



C. DISCORS, Linnaeus. 

 C. discors, Brit. Moll. ii. p. 195, pi. 45. f. 5, 6, pi. 48. f. 5. 



C. COSTULATA, RisSO. 



C. costulata, Brit. Moll. ii. p. 205, pi. 45. f. 1. 

 C. FABA, Muller. 

 C. faba, Brit. Moll. iv. Appendix, p. 256. 



Of the above, C. costulata is a variety of C. discors, though 

 admitted by authors as a species. The C. faba, taken from a 



