OF NEW SOUTH WALES. 57 



hidden by the shell. When the animal moves this foot bears on 

 the upper part of the posterior side an operculum which is always 

 horny, blackish, pauci-spiral and with a lateral nucleus. This 

 operculum forms from two and a half to three whorls ; it is semi- 

 circular, and has a straight internal edge like the same organ in 

 the genus Natica. The foot is very slightly projecting in front, 

 where it is rounded. The head is rather solid, prolongated into 

 a conical muzzle and terminated by a longitudinal slit wherein is 

 placed the mouth ; the head bears two long pointed conical ten- 

 tacles behind, broad at the base and having at the external side 

 of this base a rather salient, blunt, ocular tubercle. The shells 

 of the genus Littorina are easily distinguished from either 

 Turbo or Trochus because they are never nacreous, and besides 

 the form of the aperture, the flattened and almost trenchant 

 columella, they have peculiar characters of their own. The 

 only difficulty there would be is in separating them from 

 some species of the genus Phasianella, if one omits to observe at 

 first that in the latter genus the shells are always very highly 

 polished, and that the operculum is calcareous. Those Littorince 

 which approach nearest to Phasianella have the columella almost 

 straight and trenchant at its edge, which is never seen in the 

 latter genus. Finally the animals are different ; the Phasianelloe 

 in the ornaments of the head and the tentacles of the foot do 

 not differ from the animal of Trochus, while the Littorince, as we 

 have explained, have characters peculiar to themselves, and 

 which approaches the animal of Scalaria. Between the 

 opercula of the genus Littorina and Scalaria there is a good 

 deal of analogy. The animal of Scalaria has the head probos- 

 cidiform, the tentacles are more obtuse, shorter in proportion, 

 and the ocular tubercles are a trifle more elevated." 



Having premised these particulars, M. Deshayes defines his 

 genus thus : — Gren. Littorina, Ferussac. General characters : 

 Animal spiral, moving on a foot thin oval or sabcircular ; head 

 proboscidiform, mouth terminal, anterior ; two conical tentacles, 

 pointed, broad at the base ; eyes large, hardly projecting from 

 the external base of the tentacles ; operculum horny, pauci-spiral 



b 



