GASTEKOrOBA. 283 



Shell composed of oiglit transverse imbricating plates, lodged 

 in a coriaceous 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 limpet-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 limpet ; proboscis 

 armed with cartilaginous jaws, and a long linear tongue; 

 lingual teeth 3 ; median small, laterals large, with dentated 

 hooks; uncini o, trapezoidal, one of them erect and hooked. 

 No eyes or tentacles. Branchiae forming a series of lamellae 

 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; there-productive 

 organs are symmetrically repeated on each side, and have two 

 orifices ; the intestine is straight, and the anal orifice posterior 

 and median, 



Distrihution. 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 — 'lb 

 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 Chile e. 



Fossil, 37 species. Silurian — . Britain, Belgium, &c. 



Sub-genera* Chiton. Synonyms, Lophunis, Poli. Eadsia, 

 Callo-chiton, Ischno- chiton, and Lepto-chiton. (Oray.) 



Example, C. squamosus. PI. XI., Pig. 28. Border tessel- 

 lated. 



Distrihution, Brazil, "West Indies, Newfoundland, Greenland, 

 Britain, Mediterranean, Cape, Philippines, Australia, New 

 Zealand, West America. 



Tonicia, Gray. C. elegans. Margin bare. 



Distribution, Greenland, Cape Horn, New Zealand, Valparaiso. 



Acanthopleura, Guilding. C. spinosus. PI. XL, Pig. 29. 

 Margin covered with spines, or elongated scales. 



» The sub-genera of Dr. Gray are founded on the form of the plates of insertion ; 

 they are described in detail in the proceedings of the Zi ological Society. Dr. 

 Middendorf employs the number of the l/ranchial lamince fur distinguisliing the 

 sections. 



