CRENELLA. 63 
filamentary mass issues, and fixes the animal to Ascidie and 
marine substances. The anterior part of the foot is white, 
narrow, finger-shaped, and moderately pomted; 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 spmning 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 pot kept in movement as if searching or feeling. There 
are a pair of branchial lamin 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 im the perfect symmetry of the four branchial 
plates. 
This species is often attached to old bivalves and masses of 
Serpule, but is more usually imbedded in the coriaceous 
mantle of the Ascidia mentula, from which twenty of all sizes 
have been extracted. It imhabits plentifully the coralline 
districts at Exmouth. 
We have not seen alive the following species :— 
C. nigra, Gray. 
C. nigra, Brit. Moll. 1. p. 202, pl. 44. f. 5, and (animal) pl. Q. f. 7. 
C. pecussata, Montagu. 
C. decussata, Brit. Moll. 11. p. 210, pl. 45. f. 2. 
C. rHomBEA, Berkeley. 
C. rhombea, Brit. Moll. 11. p. 208, pl. 45. f. 3. 
C. piscors, Linnzeus. 
C. discors, Brit. Moll. 11. p. 195, pl. 45. f. 5, 6, pl. 48. f. 5. 
C. costunata, Risso. 
C. costulata, Brit. Moll. ii. p. 205, pl. 45. f. 1. 
C. rapa, Miller. 
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 
