212 SHELLS AND SHELL-FISH. PART 1. 



analysis may, therefore, be interesting, as the most 

 perfect we have been able to lay before the reader. 



(197.) The animals of the present family widely 

 differ, both in habits, shape, and anatomy, from those 

 inhabiting the TnrbidfP. In the first place, their mouth 

 is mor& like that of the slugs and snails (^Helicidcp), 

 being composed of two short lips, — the upper, and some- 

 times the under, of which is cleft and fringed ; the 

 eyes, instead of being supported, as in the Turbid fP, half 

 ^vay on the tentacula, are seated on two short but very 

 thick tubercles ; while the long and slender tentacula 

 are nearly cylindrical, and of equal breadth throughout : 

 the Avhole structure, in short, is intermediate between 

 the animals of the ear-shells (^Haliotidce) and the 

 ? lails (^Helicidce^ ; further, the generality have three 

 long slender filaments on each side of the foot, as long 

 as the tentacula, but the use of which is unknown. In 

 one of the typical groups, and in PhasmiieUa, the 

 aperture is closed by a strong, and often very thick, 

 shelly operculum ; but in the Trochidce, this covering 

 is horny, except in that particular group which con- 

 nects them with the Senectince : in the Rofellince, again, 

 the operculum is horny. 



(198.) The shells, in their typical examples, may in 

 general be recognised both by their pyramidical shape, 

 and by theh- substance being perlaceous, — a fact always 

 indicated by the rich pearly hue of the aperture. As 

 this is the most prevalent, it is perhaps the best cha- 

 racter for the mere conchologist to go by : and yet this 

 will not serve in all cases, because the pheasant -snails 

 (^Phasiianella Lam.) and the carriers (Onustiis Hump.) 

 are not pearly ; and even the most aberrant sub-genera 

 in Senectus, Trorhtis, and Monodonta, which represent 

 the carriers, are equally destitute of this substance. 

 Nevertheless, all these, excepting the first, have their 

 basal whorl so much depressed or flattened, and their 

 shape so trochiform, that a little attention will soon 

 make the student familiar with them. 



(199-) ^^ e think the following groups are the pri- 



