CHITONID. 341 
become propped outside ; sinus a mere wave in the united bodies 
of the sutural lamine; mantle reticulated with peculiar bodies, 
the tips of which appear like diamond-shaped scales, and which 
are unlike the armature of any other Chiton. C. levis, Mont. 
(Ixxxv, 73). 
STEREOCHITON, Cpr. Girdle coriaceous, sparsely downy. S. 
castaneus, Wood. 
TONICELLA, Carpenter. Valves, mucro, laminz and sinus 
usually as in Ischnochiton; zone as in Tonicia, coriaceous, 
smooth or nearly so. The genus Tonicia, Ads. and Gray, to 
which the species of Tonicella have often been referred, has 
pectinated insertion-plates and ambient gills like the typical 
Chitons, while Tonicella has sharp plates and short rows of 
gills. The two groups also differ in dentition. Distr.—Mostly 
Northern; Atlantic, California. 7. marmoreus, Fabr. 
SCHIZOPLAX, Dall. (Tonicia sp., H. and A. Adams.) Shell 
and zone like Tonicella; central valves with a median slit. 
Branchiew subambient. Distr—sS. Brandtii, Midd. Alaska, 
Siberia. 
LEPTOPLAX, Carpenter. Valves thin, smooth, partly immersed ; 
insertion-lamine acute, terminals few-fissured, but regular; sinus 
not dentate; mucro median. C. coarctatus, Sowb. (1xxxv, 74). 
May be considered a partially covered Tonicella. 
CHZTOPLEURA, Shuttleworth. (Acanthopleura, Gray.) Shell 
like Ischnochiton ; insertion-plates regular, branching; middle 
valve with one, anterior and posterior with many slits; teeth 
sharp, normal; eaves moderate; sinus broad,or minute; girdle 
hairy. Gills encircling. Distr.—Several sp. C. Perwvianus, 
Lam. (Ixxxv, 75). 
MAUGERELLA, Cpr. Middle valve with two slits; eaves pro- 
jecting ; girdle with short, striated, shelly bristles. C. conspicuus, 
Carp. 
SPONGIOCHITON, Carpenter. Valves partly immersed ; laminze 
acute, Ischnoid; sinus large, smooth; mucro median, planate ; 
zone spongiferous, produced in front. May be considered a 
partially covered Cheetopleura. C. productus, Cpr. 
ISCHNOCHITON, Gray. (Lepidopleurus, H. and A. Adams.) 
Shell thin; laminz of insertion regular, acute, neither pectinate 
nor serrate; eaves (subgrunde) large; sinus usually smooth ; 
girdle squamose, the squamz generally striate. Branchie 
elongated. @. longicymba, Blainv. (1xxxv, 76). 
The main character of this group, which includes by far the 
largest number of species of any single group of Chitons, consists 
in the row of sharp, smooth insertion-teeth, surrounded by more 
or less projecting eaves, and in the scaly girdle. 
Dr. Carpenter has proposed the following eight subgenera, 
which [ retain as sections : 
