388 TRocriiDit:. 



times ciliated; eyes on free peduncles at their outer 

 bases ; two more or less developed head-lobes between 

 the tentacles. Gill single, long, and linear. Sides of 

 the foot with a large neck-lappet near the eye-peduncle, 

 continuous with a conspicuous side-membrane bearing 

 on its free margin from three to five tapering filaments ; 

 operculigerous lobe often ornamented with cirrhi. 



Operculum hornj^, spiral, often with a solid, convex, 

 calcareous coat, rarely wanting. 



Shell pyramidal, turbinate, or ear-shaped ; aperture 

 pearly within. 



The Trochoid Scutibranchs embrace an extensive 

 series of herbivorous, littoral Mollusks, characterised by 

 the fringed lobes and tentacular cirrhi of the head and 

 sides, their pedunculated eyes, and by the pearly nature 

 of their shells, which exhibit a brilliant nacre when the 

 epidermis and outer coat are removed. They are in- 

 variably marine, feeding on the sea-weeds which abound 

 along the shore and are distributed universally in all 

 parts of the globe, being most numerous, and of larger 

 growth and more beautiful colours in tropical seas. The 

 shells of the TrocliiiJcB, though formed on one type, 

 assume great variety of contour and sculpture, being 

 turbinate and provided with stony opercula in the Eic- 

 tropibnc and Ti(rhmin(e ; cancellated and discoidal, with 

 corneo-calcareous opercula in the LiotiiiKe ; conical or 

 pyramidal, with horny, multispiral opercula in the Tro- 

 chince; and ear-shaped, with the opercula rudimentary or 

 wanting in the 8tomateUin(B, which section gradually 

 leads to the family of Haliotidie, in which the branchial 

 plumes are symmetrical, the muscle of attachment is 

 central, and the mantle is fissured in front. 



