GASTEROPODA. 283 
Shell composed of eight transverse imbricating plates, lodged 
- in acoriaceous mantle, which forms an expanded margin round 
the body. The first seven plates have posterior apices; the 
eighth has its apex nearly in front. The six middle plates are 
each divided by lines of sculpturing into a dorsal and two 
lateral areas. All are inserted into the mantle of the animal 
by processes (apophyses) from their front margins. The 
posterior plate is considered homologous with the lmpet-shell 
by Dr. Gray; the other plates appear like portions of its 
anterior slope, successively detached. The border of the mantle 
is either bare or covered with minute plates, hairs, or spines. 
Animal with a broad creeping disk like the hmpet; proboscis 
armed with cartilaginous jaws, and a long hnear tongue; 
lingual teeth 3; median small, laterals large, with dentated 
hooks; uncini 5, trapezoidal, one of them erect and hooked. 
No eyes or tentacles. Branchize forming a series of lamelle 
between the foot and the mantle, round the posterior part of 
the body. The heart is central, and elongated like the dorsal 
vessel of the annelides; the sexes are united; the re-productive 
- organs are symmetrically repeated on each side, and haye two 
orifices; the intestine is straight, and the anal orifice posterior 
and median, 
Distribution. More than 250 species are known; they occur 
in all climates throughout the world; most abundant on rocks 
at low water, but frequently obtained by dredging in 10—25 
fathoms. Some of the small British species range as deep as 
100 fathoms. (Forbes.) West Indies, Europe, South Africa, 
Australia, and New Zealand, California to Chiloé. 
Fossil, 37 species. Silurian—. Britain, Belgium, &c. 
Sub-genera.* Chiton. Synonyms, Lophurus, Poli. Radsia, 
Callo-chiton, Ischno-chiton, and Lepto-chiton. (Gray.) 
Hxample, C. squamosus. Pl. XI., Fig. 28. Border tessel- 
lated. 
Distribution, Brazil, West Indies, Newfoundland, Greenland, 
Britain, Mediterranean, Cape, Philippines, Australia, New 
Zealand, West America. 
Tonicia, Gray. OC. elegans. Margin bare. 
Distribution, Greenland, Cape Horn, New Zealand, Valparaiso. 
Acanthopleura, Guilding. C. spinosus. Pl. XI., Fig. 29. 
Margin coyered with spines, or elongated scales. 
* The sub-genera of Dr. Gray are founded on the form of the plates of wnsertion ; 
they are described in detail in the proceedings of the Zoological Society. Dr. 
Middendorf employs the number of the branchial lamine for distinguishing the 
secticus. 
