LITTORINA. 349 
L. PALLIDULA, nobis. 
Lacuna pallidula, Brit. Moll. iii. p. 56, pl. 72. f. 1,2, and f. 3, 4 (as 
patula). 
Nerita pallidula, Da Costa. 
Animal spiral, nearly white throughout; mantle simple, 
tumid at the margin, but even with the aperture of the shell. 
The head is a cylindrical produced annulated muzzle; the 
upper part of the neck has two short flake-white diverging 
Imes imbedded in the ground colour ; the disk is transversely 
oval with a vertical fissure, within which the white spiny 
tongue can be seen in action. The tentacula are long, setose, 
and taper conically to their termini, with rather small eyes 
raised on short external offsets. The foot is always white 
below, pale drab or yellowish-white, or white, above, of oval 
shape in quietude, when on the march oval-clongated, rounded 
anteally and posteally, with occasionally sight emarginations 
behind, very considerably contracted at the anterior third of 
its length, with a pale flaky border; the other two-thirds 
have an intense flake-white margm. ‘There is the central 
longitudinal fissure or groove that gives the animal the undu- 
latory quality of progression, by alternate halves of the foot, 
which action is the principal generic characteristic of the true 
Littorine. The thin upper membrane that bears the oper- 
culum extends nearly to the junction of the foot with the 
body; the anterior terminus of the foot is slightly grooved, 
forming a sort of upper and under lobe, or pair of shallow 
labia; the operculigerous lobe, is expanded laterally beyond 
the pedal limits into minute wing-like processes, and at the 
terminal pomt is subcircularly scalloped out; the lateral 
margins forming usually two, sometimes three or four very 
short, white, caudal fillets of different lengths, variously shaped, 
but usually compressed and slightly triangular; these are 
occasionally in the same species either rudimentary or quite 
obsolete. The single light brown respiratory plume can, in 
certain positions of the animal, be observed, branching from 
left to right ; it has 35-45, or more, long, slender pectinations ; 
there may also be seen, without dissection, the short white 
termination of the rectum, accompanied by the excretory 
