CHAP. IX. SCUTIBRANCHIA, OR LIMPETS. 241 



the typical Turbinellidce, so that it is impossible to de- 

 termine what are its relations to the Trochidce. In the 

 present state of malacology, our only surprise should he 

 that so much may be effected, and so many land-marks 

 planted, with such very scanty materials. 



CHAP. IX. 



ON THE ABERRANT TRIBES OF THE GASTEROPODA, NAMELY, 



THE SCUTIBRANCHIA, OR LIMPETS; THE CYCLOBRANCHIA, OR 

 CHITONS ; AND THE PTEROPODA, OR CRYSTAL-SHELLS. 



(222.) THE immense superiority in extent and multi- 

 plicity of types, which the phytophagous and carnivorous 

 gastropods possess over all the other Testacea, not to 

 mention the superior interest attached to them by con- 

 chologists and collectors, have induced us to devote the 

 greater part of our volume to their illustration. Our 

 space being limited, we have therefore no other resource 

 but to abridge our remaining survey as much as possible. 

 (223.) The first of our aberrant tribes is composed 

 of the SCUTIBRANCHIA, or limpets ; in extent, it is little 

 larger than one of the least families of the spiral shell- 

 fish: they may be almost termed sedentary or fixed ; for 

 although some have the power of locomotion, it is so 

 limited that their shell often assumes the contour of the 

 spot where they have remained. In the arrangement of 

 their branchia, and the form of their bodies, they may 

 almost be called testaceous Doridce, at least so far as the 

 typical species are concerned. The passage to this tribe, 

 from the Haliotidce, or ear-shells, is rendered perfectly 

 easy and natural, whether we regard the want of a spire in 

 Crepidulaor the patelliform shape of Calyptraa. We have 

 now arrived at the last division of univalve shells, where 

 the structure is so simple that the shelly part of these 



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