Mr. W. Clark on tJie Lacunse. 31 



other angle is produced into a yellow, conical, arcuated, pointed, 

 rather long process, the only use of which would appear to be 

 that of an organe excitateur. 



Lacuna, Turton. 



Nerita pallidula, Da Costa. 



Littorina pallidula, mihi. 



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 ofi'sets. The foot 

 is always white below, pale drab or yellowish white, or white, 

 above, of oval shape in quietude, when on the march oval-elon- 

 gated, rounded anteally and posteally, with occasionally slight 

 emarginations behind, very considerably contracted at the ante- 

 rior third of the 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 

 Littorina. The thin upper membrane that bears the operculum 

 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 opercu- 

 ligerous lobe is expanded laterally beyond the pedal limits into 

 minute wing-like processes, and at the terminal point is sub- 

 circularly scalloped out ; the lateral margins forming usually two, 

 sometimes three or four very short, white, caudal fillets of dif- 

 ferent lengths, variously shaped, but usually compressed and 

 slightly triangular; these are occasionally in the same species 

 either rudimentary or quite obsolete. The single hght brown 

 respiratory plume can in certain positions of the animal be ob- 

 served branching from left to right ; it has 35-45 or more long, 

 slender pectinations ; also there may be seen, without dissection, 

 the short white termination of the rectum, accompanied by the 

 excretory canal of the sac that contains the viscous fluid. The 

 neck of the animal is simple and free from membranous lappets, 

 as are all the Littorina. The fsecal pellets are elongated, slender, 

 Bubcylindrical, having the apices tipped with a dark hue. The 



