LITTORINA. 345 
greater circumspection in future in the creation of species, 
the object I have had in view will be accomplished. 
L. nerirorpeEs, Linnzus. 
L. neritoides, Brit. Moll. iii. p. 26, pl. 84. f. 1, 2. 
Turbo petreus, Montagu et aliorum. 
Animal spiral; mantle even with the shell. The head is a 
long proboscidiform cloven muzzle, the upper part of an 
intense black cloud-colour; orifice of the mouth white, with 
a vertical fissure; tentacula awl-shaped, moderately long, 
flattened ; eyes large, not on pedicles, but placed on the sub- 
stance of the skin at the bases of the tentacula, inclining 
externally only in a triflmg degree. The buccal mass is plain 
brown, supported by two thin coriaceous plates of the same 
colour, from whence a very long white spiny tongue proceeds 
to the stomach, and there lies coiled as in Littorina littorea ; 
but it is proportionately longer than in that species, being 
2 inches long. Foot nearly as in L. littorea, very slightly 
auricled and curved in front, rounded posteriorly to a terminus, 
which is a little jagged or dentated, forming an oval when not 
in action, but on the march a very elongated oval; above, its 
colour is black; underneath, a pale lead ground, mixed with 
two shades of white and one of purple. These colours are 
divided into three portions ; the anterior one is the narrowest, 
of an intense hyaline-white; the middle is also hyaline, and 
the third is hyaline pale purple. The foot is not strictly 
divided into two longitudinal half-parts as in L. littorea, but 
at the anterior part, where the intense hyaline-white termi- 
nates, appears transversely broken or furrowed, so as to allow 
of a subdued alternate undulatory gait, or quality of progres- 
sion, something like that of L. littorea, in which the whole of 
the longitudinal half is first advanced, and then the other; 
but here only half of the anterior part of the foot is moved 
forward, and then the other. 
This alternate action of parts of the foot is a very singular 
character, which obtains, more or less, in all the true Littorine, 
and is with very few exceptions confined to the genus. There 
is only one branchial plume, and the mternal and external 
