CHITONID^. 341 



become propped outside ; sinus a mere wave in the united bodies 

 of the sutural lamina? ; mantle reticulated with peculiar bodies, 

 the tips of which appear like diamond-shaped scales, and which 

 are unlike the armature of any other Chiton. G. Isevis, Mont. 

 (Ixxxv, 73). 



STEREOCHiTON, Cpr. Girdle coriaceous, sparsely downy, S. 

 castaneus^ Wood. 



TONiCELLA, Carpenter. Yalves, mucro, laminae and sinus 

 usually as in Ischnochiton ; zone as in Tonicia, coriaceous, 

 smooth or nearlj^ so. The genus Tonicia, Ads. and Gray, to 

 which the species of Tonicella have often been referred, has 

 pectinated insertion-plates and ambient gills lil^e 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. T. mai^moreus^ Fabr. 



scHizoPLAX, Dall. (Tonicia sp., H. and A. Adams.) Shell 

 and zone like Tonicella ; central valves with a median slit. 

 •Branchiae subambient. Distr. — S. Brandtii, Midd. Alaslva, 

 Siberia. 



LEPTOPLAX, Carpenter. Valves thin, smooth, partly immersed ; 

 insertion-laminas acute, terminals few-fissured, but regular ; sinus 

 not dentate; mucro median. G. coarctatus, Sowb. (Ixxxv, '74). 

 May be considered a partially covered Tonicella. 



CH^TOPLEURA, 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. G. Peruvianiis., 

 Lam. (Ixxxv, 75 ). 



MAUGERELLA, Cpr. Middle valve with two slits ; eaves pro- 

 jecting ; girdle with short, striated, shelly bristles. G. conspicuus^ 

 Carp. 



SPONGIOCHITON, Carpenter. Yalves partly immersed ; laminae 

 acute, Ischnoid ; sinus large, smooth ; mucro median, planate ; 

 zone spongiferous, produced in front. May be considered a 

 partially covered Choetopleura. G.productus^ Cpr. 



iscHNOOHiTON, Gray. (Lepidopleurus, H. and A. Adams.) 

 Shell thin ; laminae of insertion regular, acute, neither pectinate 

 nor serrate ; eaves (subgrundge) large ; sinus usually smooth ; 

 girdle squamose, the squamee generally striate. Branchiae 

 elongated. G. longicyviba, Blainv. (Ixxxv, 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 I retain as sections ; 



