LITTOEINA. 349 



L. PALLIDULA, nobis. 



Lacuna pallidula, Brit. Moll. iii. p. 56, pi. 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 

 lines 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-elongated, rounded 

 anteally and posteally, with occasionally slight 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 margin. 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 

 Littorints. 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 point 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 wliite 

 termination of the rectum, accompanied by the excretory 



