204 CHITONID.E. 



system is complicated; Cuvier ascertained that each 

 auricle opened into the heart by two distinct orifices_, a 

 disposition of which he had not detected another instance 

 in the animal kingdom. Milne-Edwards considered 

 them a satellite group of the ^lollusca, fancifully com- 

 paring the organization of the Invertebrata to the side- 

 real system. But the general plan of their structure 

 is that of the limpet ; the only differences of any import- 

 ance consist in the latter having tentacles and eyeSj which 

 are wanting in the ChitonidcB, and in the shell of the one 

 being a single piece, while the other is composed of 

 several pieces which form together an elongated buckler. 

 In the genus Cylichna we find one species (C truncata) 

 with tentacles, and another (C. cylindracea)^\i]iovX ten- 

 tacles ; and in each of the genera Eulima, Mangelia, and 

 Amphisphyra, similar discrepancies occur with respect to 

 the presence or absence of eyes in certain species. The 

 most obvious distinction between Chiton and Patella con- 

 sists in the arrangement of the gills and the multivalve or 

 univalve character of the shell. It seems sufficient to 

 group them in two families, separate but not widely 

 apart. Adanson and Strom pointed out the affinity of 

 Chiton to Patella ; and Poli showed that their spinous 

 tongues were exactly similar. The Rev. Lansdown 

 Guilding, in a valuable monograph of the present family 

 (Zool. Jourm 1830), called this apparatus ^^trachy- 

 derma.^' 



Genus CHI'TON^ Linne. PI. V. f. 2. 



Body oval or oblong : girdle scaly, bristly, tufted, or mem- 

 branous, and fringed with short spines. 



SuELL usually boat-like, composed of eight plates, which are 

 * Coat of mail. 



