392 MOLLUSCA. 



been found to a depth of 2000 fathoms. They are known fossil 

 since the Silurian. 



There is only one family, the Chitonidae, but there is considerable variation 

 in the group, both specific and individual, and consequently a large number 

 of species has been described. These have been arranged in genera, of which 

 there are a considerable number ; but the attempt which has been made by some 

 authors to arrange these genera in several families, and these again in sub-orders, 

 seems to be going further than the facts warrant, for the uniformity in structure 

 in the group is remarkable, and the differences, though numerous, are trivial 

 and unimportant. 



Fam. Chitonidae. With characters of sub-class. Chiton L. plates of shell 

 much exposed; Leptochiton Gray, edge of mantle uniformly covered with scale- 

 like spicules ; Callochiton Gray, with branchiae only in the hinder part of the 

 mantle-groove; Acanthochiton Leach, edges of mantle with spicules united in 

 bundles corresponding to the plates of the shell ; Plaxiphora Gray ; Cryptoplax 

 Blainville (Chitonellus Lam.), shell-plates largely concealed and not articulated 

 together, foot narrow ; Cryptochiton Middendorf, shell-plates completely em- 

 bedded in the mantle. 



SUB-CLASS 2. ANISOPLEUKA. 



Asymmetrical Gastropoda with shell of one piece, never more 

 than two ctenidia, and with a well-marked visceral commissure. 

 Generally with a veliger larva. 



The above definition contains the main points in which the rest 

 of the Gastropoda differ from the Isopleura. The Anisopleura are 

 divided into two orders, the Streptoneura and the Euthyneura. 



Order 1. STREPTONEURA.* 



Dioecious Anisopleura with shell and generally with operculum; 

 with gills in front of the heart. Visceral commissure twisted into 

 a Jigure-of-8. 



Behind the head, which is usually distinct and bears a single pair 

 of tentacles, lies the mantle-cavity, into which the rectum, kidney, 

 and oviduct open. In rare cases two gills are present, but as a 

 rule the right gill is absent. The branchial veins enter the heart 

 from the front. The visceral commissure is twisted into a figure- 

 of-8, of which the right half passes dorsal to the alimentary canal, 



* E. Claparede, "Anatomie u. Entwick. d. Neritina fluv.," Z.f. w. Z., 2, 1850. 

 H. Lacaze Duthiers, "Sur le systeme nerv. de 1'Haliotide et sur la Poupre," Ann. 

 Sci. Nat. (4), 12, 1859. Id., "Sur I'anat. et 1'embryog. des Vermets," Ann. 

 Sci. Nat. (4), 13, 1860. E. L. Bouvier, " Systeme nerv., inorph. gen. et classi- 



Zool. Exp. (2), 2, 1884. Boutan, "Sur I'anat. et le devel. de la Fissurella," Arch. 

 Zool. Exp. (2), 3 bis, 1886. 



