LECTURES ON MOLLUSCA. 221 



spreads out below, and gills between the upper and lower portion. 

 These gills are not like the single long gill of the Limpet, curled 

 round; but are two long, symmetrical organs; it being the fashion of 

 Chitons to double almost everything, the generative orifices included. 

 The head gradually becomes smaller in proportion, is covered with 

 granules which become the eighth valve, and develops a slit, which 

 becomes the mouth. It then loses the eyes; the head never stretches 

 beyond the valves, and there are no tentacles. 



The Chitons live chiefly on rocks and under stones at low water and 

 in moderate depths. They are sluggish creatures, and apparently 

 neither disturb others or are themselves disturbed, (except by conchol- 

 ogists.) They are found in all seas ; but the finest species are not 

 found in the tropics. The largest are from the colder western rocks 

 of North and South America. Different as the Chitons are from all 

 other living creatures, they are very like each other. The different 

 groups are not generally confined to particular shores ; but the species 

 do not travel so far as Limpets and ordinary mollusks, as, indeed, we 

 might suppose from the young having no swimming fins. A large 

 number of genera have been proposed by modern authors, of which 

 the following are the principal; writers unfortunately not agreeing on 

 the group for which the old name should be retained. 



The true Chitons have the mantle covered with smooth scales, and 

 the end valves elegantly pectinated at the edge ; the back valves hav- 

 ing the apes raised. Enoplochiton has the scales long and unequal ; 

 the back valve with smooth edge and depressed apex. In general the 

 middle valves have only one notch ; but in Raclsia there are two ; and 

 in Callochiton, the edges are cut into four bifid lobes. In Lepidopleu- 

 rus the valves are thin, and easily fall off; the insertion-plates being 

 inside the colored parts. The mantle-scales are extremely small. In 

 Leptochiton, which includes most of the northern forms, the scales are 

 minute, the gills short, and the insertion-plates rudimentary, with- 

 out notches. In Lorica and Schizochiton, the mantle and last valve 

 are slit behind. They have very minute scales, and in the latter group 

 the valves are very small as compared with the mantle. 



In the next series, the mantle is covered with thick hairs or bris- 

 tles. Acanthopleura has the insertion-plates pectinated. Corephium 

 has the mantle-spines shelly, and the back valve not lobed at the 

 sides. Mopalia has the mantle much produced in front, and narrowed 

 behind. 



A comparatively small group Tonicia has the mantle naked and 

 smooth. One species, in which the valves are more separated, has 

 been dignified by Dr. Gray with the classical generic name Fannyia. 



The Oregon district produces a curious group of Chitons, in which 

 the valves are nearly or entirely covered by the fleshy mantle. The 

 commonest species, whicli was first sent to the British Museum by 

 Lady Katherine Douglas, and therefore called by Dr. Gray Katherina 

 Douglasice, (Anglice, Douglas's Catherine,) has the valves partly 

 exposed and the skin smooth. The giant Crypfochiton, the anatomy 

 of which has been so carefully described by Dr. Middendorff, has 

 gritty particles in the rough skin. There is no sculpture on the valves, 



