CHAP. IX. SCUTIBRANCHlAj OR LIMPETS. 



245 



differs in having a callous prominent rim placed longi- 

 tudinally on one side only of the inner surface, and 



to which the principal muscle is pro- 

 bably attached : there is no regular 

 apex, but merely a rudimentary indi- 

 cation of one, — one side of the shell 

 being more gibbous than the other ; 

 so that, if this faint indication of a vo- 

 lution had formed an apex, it would 

 have been longitudinal, or on the late- 

 ral sides of the shell, — and not central, as in Patella. The 

 annexed figures will illustrate this description, and show 

 the irregularity of form in one and the same species. We 

 found all these adhering to coral fished up on the coast of 

 Sicily, so far back as 1 807 ', and a few specimens, if we 

 mistake not, were given to our friend Dr. Leach, for the 

 British Museum. It is perfectly analogous to the parasitic 

 genus Coccus among insects; and, indeed, much more 

 resembles them, or the internal shell of some unknown 

 tectibranchian moUusk, than any thing else. An approxi- 

 mation is made to this genus by certain small limpets, 

 whose apex, from being lateral, makes them also inequi- 

 lateral shells : of course, we know nothing of the animal ; 

 but, from the nature of the shell, it must be perfectly 

 sedentary, as it embraced the coral most closely. 



(229.) The Cyclobranchia, or Chitones, are known 

 at once by their shells. They differ from all the gastro- 

 pods yet noticed *, in being quite destitute of tentacula, 

 — these organs being supplied by a membranaceous veil 

 round the mouth : the mantle, also, unlike that of the 

 Patellce, is always exposed, and forms the circumference 

 of that part by which the animal adheres : it is therefore 

 externally coriaceous, and is covered for its protection 

 by minute scales resembling shagreen, and often with 

 spines or setaceous hairs ; and on each side, underneath, 

 are the lamellar branchia : this arrangement of the or- 

 gans of breathing gives the name to the tribe. The 



* Except Herpa among the Limachue, which is the cyclobranchian type 

 of the HelicidiE. 



B 3 



