LEPTOPLAX. 25 



Genus X. CALLOCHITON Gray. 



Valves exposed ; insertion plates cut into numerous teeth rising 

 from very spongy eaves, the teeth more or less buttressed at the out- 

 side edges. Sutural-plates continuous across the very shallow median 

 sinus. Girdle typically covered with fine compact diamond-shaped 

 scales ; but in the subgenus Stereochiton sparsely downy, frequently 

 naked by erosion. 



Genus XI. ISCHNOCHITON Gray. 



Valves exposed ; insertion plates sharp, smooth ; eaves not 

 spongy, or rarely so. Umbo of posterior valve varying from pos- 

 terior to anterior. Girdle covered with imbricating scales. 



Genus V. LEPTOPLAX Carpenter, 1882. 



Leptoplax CPR. MS., and Table of Kegular Chitons, 1871. DALL, 

 Proc. U. S. Nat. Mus. 1882, p. 286. Type C. eoarctdtus Sby. 



Valves thin, partly immersed in the thin, smooth girdle. Inser- 

 tion plates acute, with few slits, but regular. Sinus scarcely toothed ; 

 mucro median. (Qjr.) 



The prominent characters of this genus are, (1) valves partly 

 covered by a thin extension of the girdle, (2) girdle smooth, as in 

 Tonicella, (3) insertion plates long, with few slits. The gills are 

 unknown. 



L. COARCTATUS Sowerby. PL 11, figs. 38, 39. 



Shell elongated, very delicate, little elevated, the dorsal ridge 

 acute ; greenish-ashen or reddish-olive. Valves of the normal form, 

 but two-thirds covered by a thin cuticle continued from the girdle. 

 Exposed portion of the valves flask-shaped, the jugular area smooth, 

 subelevated, having longitudinal rows of points; central areas and 

 end valves having large pustules, close and somewhat radiately 

 placed, about 20 on each side in a young specimen, 100 in an adult ; 

 lateral areas scarcely defined ; mucro slightly behind the center, 

 rather elevated. Interior : Plates of insertion very long, rose- 

 tinted, here and there delicately striated outside. Slits small, in the 

 posterior valve 6, posteriorly situated ; central valves 1, anterior 

 valves 4, having shallow grooves running up to the eaves. Sinus 



