340 CHITONIDA. 
CYMATOCHITON, Dall. Leptochiton with the valves thrown for- 
ward. Differs from Proboleum in the valves being transverse 
instead of squared, and in the terminal valves being regular 
instead of waved. C. Loftusianus, King (1xxxv, 68, 69). 
Recent Forms. 
LEPTOCHITON, Gray. (Lepidopleurus, Risso. Craspedochilus, 
and Boreochiton, Sars.) Insertion-laminz none; girdle or zone 
minutely sandy; sinus smooth, not laminated. Branchize short. 
Distr.—10sp. Northern Seas, Kerguelen’s Isl. Type, L. asellus, 
Lowe. L. cancellatus, Sowb. (Ixxxv, 71). Only 2 out of the 
25 species cited by Adams really belong here. 
DESHAYESIELLA, Carpenter. Valves curved forwards ; mucro 
planate; zone or girdle spiculose ; insertion-plates none ; sutural 
lamine triangular, raised. C. (Leptochiton) curvatus, Cpr. 
HANLEYIA, Gray, Anterior valve with an unslit insertion- 
plate; other valves destitute of the plates. Dzstr.—Mostly 
northern; one from deep waters, Gulf of Mexico. C. debilis, 
Gray. 
HEMIARTHRUM, Carpenter. Insertion-plates present on all the 
valves, but entire without slits. H. setuloswm, Carpenter. 
MICROPLAX, H. Adams. Resembling Chitonellus externally ; 
submerged laminee unslit, entire, fused in an undistinguishable 
manner with the parts which usually constitute the sutural 
lamine. MM. Grayi, Ad. and Ang. 
B. Ischnoidea. 
Insertion-plates sharp, smooth, fissured; with eaves. 
* No pores on girdle. 
TRACHYDERMON, Cpr. (Lepidopleurus sp., Auct. Craspedo- 
chilus, Sars.) Insertion-laminze acute, smooth; valves within 
and without exactly like Ischnochiton ; zone or girdle not porif- 
erous, granulated by very minute scales. Branchiz short. This 
name was originally proposed as a subgenus of Ischnochiton to 
include Gray’s second section, ‘‘mantle-scales minute, granular.”’ 
In all other conchological characters the group accords with that 
genus, but the animal differs in having the gills either entirely 
posterior or reaching forward from the tail only to about. the 
middle of the foot, while in Ischnochiton and Chiton they travel 
to its anterior extremity. These characters indicate a transition 
between the Ischnoid and Leptoid Chitons by means of Trachy- 
dermon and Tonicella. Distr.—Chiefly northern. C. ruber, 
Lowe (Ixxxv, 72). 
TRACHYRADSIA, Cpr. Central valve doubly or many-cleft. . C. 
fulgetrum, Reeve. 
CALLOCHITON, Gray. Taming broken up into very numerous 
teeth rising out of spongy eaves, and having a tendency to 
