LECTURES ON MOLLUSC A. 63 



We have now completed our sketch of the Comb-gilled Crawlers ; 

 the largest, and (except the Cuttles) the most highly organized group 

 of mollusks. In the next order, the gills consist of two series of plates, 

 more like those of the bivalves. This comparatively trifling distinc- 

 tion is found to he coordinate with an inferior type of development in 

 other points of structure. The animals, while often much more orna- 

 mented than in the former order, are not as it were so concentrated. 

 There is never found a breathing pipe or a predacious snout. The 

 teeth, instead of being compacted into rows of 3' 1*3, each one of 

 which has its special shape, are spread out into very complex series 

 of glassy hooks, of which many in the same line are the dittos of each 

 other. The shells, while many of them are of surpassing beauty, 

 nacreous as the pearl oyster, often lose their spiral form, adopting 

 that of the simple cone. And the arrangements for the continuance of 

 the species, instead of being separated on different animals, are united 

 in the same individual, which is supposed to be capable of self-impreg- 

 nation. 



ORDER SCUTIBRANCHIATA. (Shield-giUed Crawlers.} 

 Family NERITID^;. (Nerites.) 



Almost all the Scutibranchs are shore shells,, living wherever there 

 are rocks or marine vegetation. Some are found at slight depths; a 

 few of the lower kinds only being found in deep water. 



The Nerites are almost exclusively confined to tropical shores. 

 They grub among the stones and rocks on the sea-weed, sleeping by 

 day, and prowling about, harmless as they are, towards night. They 

 are plain-looking creatures, like the Periwinkles, from which they are 

 at once distinguished by the great length of their tentacles, and the 

 eyes which rise on short stumps behind. The shells are very readily 

 distinguished by the broad flat pillar-lip and stumpy spire. Though 

 greatly abounding in species and in individuals, there are very few 

 generic forms among them. The true Neritas are strong, sea shells, 

 with stout teeth or wrinkles on the pillar lip. The operculum is sub- 

 spiral and shelly, with a stout knob fitting like a hinge under the 

 pillar lip. The Neritinas are much thinner shells, almost exclusively 

 inhabiting fresh waters, where they adhere to stones or water plants. 

 The pillar lip is thin and smooth, or only very finely toothed; the 

 operculum also is thin, with a horny edge. In the group Clithon, the 

 whirls have a row of spines pointed towards the apex. These live on 

 stony bottoms, in still, tropical waters. Some of the Neritinas, espe- 

 cially in the group Neripteron, with winged pillar lip, have very short 

 spires ; they then pass into the fossil form Velates, which is peculiar to 

 the French tertiaries. Here, while the mouth of the shell has the usual 

 Neritoid appearance, the back is conical, with only a minute spire at 

 the point. In Pileolus, a form peculiar to the oolitic rocks, there is 

 no spire at all, the back of the shell being exactly like a limpet. 

 Another oolitic form, Neritoma, has a notch in the outer lip, like 

 Pleurotomaria. A large group of fresh-water Nerites in the Bast^In- 

 dian Archipelago are limpet shaped, but with the point at the side, 



