298 MOPALIA. 



ulate ; diagonal riblets obsolete or delicately raised. Color oliva- 

 ceous obscurely mottled with dusky, the tail valve having a light 

 ray behind. 



Interior blue-green. Tail valve slightly waved upward in the 

 middle behind, and having either a simple narrow caudal slit, or a 

 slit with a small tooth set in its apex. Lateral slits of the tail valve 

 generally double on one side. Girdle narrow, sparsely hairy. 



Length 22*, breadth 12$ mill.; divergence 105 (Carpenter's 

 type). 



Length 22, breadth 13 mill. ; divergence 105 (San Diego spec.). 



Length 23, breadth 15 mill.; divergence 98 (Sta. Barbara 

 spec). 



Santa Barbara (Canfield, Gabb.) ; San Diego (Hemphill). 



Chiton acutusCpn., P. Z. S. 1855, p. 232. Mopalia acuta CPR., 

 Brit. Asso. Rep. 1863, p. 648. Mopalia plumosa and M. (f var.) 

 fissaCpR., MS. (? Not Ch.plumosus Gld). 



The comparative tenuity of the highly angled valves, their deli- 

 cate sculpturing over the whole surface, the fine denticulation of the 

 sutures and the narrow, often toothed caudal sinus, all give an 

 aspect of individuality to this form, and cause me to rank it with 

 hind*ii and lignosa as a well-differentiated subspecies. 



Carpenter, working from very few specimens, made three species 

 of the forms of this. The occasional bi-slitting of the tail plate on 

 one side is found also in some specimens of the elevated variety of 

 lignosa from Olympia, Wash. It seems to me to be a very variable 

 and trifling character. The caudal sinus seems to be indifferently 

 narrowly slit or with a little denticle in the apex of the slit, and this 

 little denticle also occurs sometimes in M. lignosa. The posterior 

 valve has a narrow light triangle behind in all the specimens I have 

 seen. 



I have no hesitation in pronouncing plumosa and fissa absolute 

 synonyms of acuta, but give here Carpenter's descriptions. 



M. plumosa. Carpenter. (PI. 64, figs. 80, 81.) Shell oval, regu- 

 lar, thin, elevated, the dorsal ridge acute, mucro behind the middle, 

 depressed. Valves subquadrate, slightly beaked behind, hardly so 

 in front. Olivaceous-ashen maculated with darker; central areas 

 (with the scarcely defined jugal tracts) reticulated-porous, being 

 sculptured with interlacing riblets forming quincuncially arranged 

 impressed pits. Lateral areas defined but hardly elevated, sculp- 



