230 SHELLS AND SHELL-FISH. PART I. 



trcea* The whole may, therefore, be characterised as 

 patelliform shells, always possessing either a small spire 

 of two or more volutions, or some internal support within, 

 although the pillar is wanting. They are, in fact, the dis- 

 coid type of the phytophagous Gasteropoda; representing 

 on one side the testaceous Cephalopoda, and on the other 

 the Scutibranchia, or limpets. Like the Volutidte, they 

 have no operculum, and both represent each other in 

 the extreme shortness of their spire. In comparison 

 to those we have already noticed, this is a very small 

 family, so small, indeed, that its primary divisions 

 are only of the rank of genera. They are, in a manner, 

 fixed or sedentary shell-fish; for, although some are 

 capable of locomotion, they must move but very little, 

 since the obvious construction of their shells t is mani- 

 festly for the purpose of adhesion. Hence they are 

 found closely affixed to rocks or other substances near 

 or within the sea. The round holes in the perforated 

 ear-shells (many species of which are of a large size and 

 splendidly iridescent) are for the passage of slender 

 filaments which the animal can protrude at pleasure : 

 these, of course, do not exist in those ear-shells which 

 are without perforations ; but in both the mantle of the 

 animal is highly ornamented, being in Stomatia regu- 

 larly cut into numerous points, like the teeth of a saw ; 

 and these, in Haliotis, assume the more lengthened shape 

 of filaments. Both these, which constitute the two 

 typical genera, enjoy the free power of locomotion, for 

 we do not observe that the contour of the shell of in- 

 dividuals of the same species ever varies, a clear proof 

 that they never take the form of that substance they 

 may happen to be attached to. In Calyptrcea and Cre- 

 pidula, however, the case is different. We have no 

 doubt that the greater part, at least, of these shell-fish 



* Cuvier, indeed, admits these and similar patelliform shells approxi- 

 mate in their animals to the Trochidce, and yet he places them widely 

 apart from Stomatia, whose structure in its soft parts is still more like 

 that of Trochus. 



t Except, of course, Sigaretus, which has its shell enveloped on its 

 back, as representing the Tectibranchia. 



