296 MOPALIA. 



In its typical state (pi. 63, fig. 46), this species may be known by 

 the strong though variable sculpture, dull brown or blackish-olive 

 color, the small sinus, the girdle being thickly set with round hairs 

 varying from strong bristles to soft hairs. A variation in color is 

 seen in some southern specimens (Tomales Bay) which are orange 

 or scarlet (pi. 63, fig. 48), or yellowish maculated with orange and 

 vermilion. Others from the same locality are vivid blue-green or 

 green (pi. 63, fig. 47). 



In his work on the Chitons of Alaska, Dr. W. H. Dall discusses 

 the synonymy and variation of this species, concluding that M. 

 lignosa Old. should be regarded as an extreme form, connected by 

 intermediate examples with the typical muscosa. He writes as fol- 

 lows : " This species can be distinguished from all varieties of 

 ivossnessenskii by its blackish and proportionately much narrower 

 girdle, and by a sort of prolongation of the external layer of the 

 shell forward under the apex of the next anterior valve in the 

 median line, forming a sort of anterior false apex, which is hidden 

 until the valves are separated. In ivossnessenskii this part is 

 squared off, the girdle is yellowish (when alive), and the valves are 

 much less transverse. It will be surprising if those who have only 

 observed these animals by a few dry specimens in collections are 

 willing to accept the synonymy above given, I confess that not long 

 since I would have been unwilling to believe that the rough, bristly, 

 typical muscosa and the dark, smooth hindsii could be properly com- 

 bined under one name with each other or with the finely reticulated 

 and painted lignosa ; but the study of a large multitude of speci- 

 mens has convinced me no arbitrary line can be drawn anywhere in 

 a fully representative series, beginning with coarsest ciliata \_muscosa~] 

 and ending with a practically smooth hindsii. The characters of 

 girdle, sculpture and form are not only variable in themselves, but 

 are found variably combined, except that it is rarer to find coarsest 

 sculpture with a downy than with a bristly girdle. However, even 

 this occurs. On the other hand, out of such a series a dozen forms 

 might be selected which, if only the characters were constant, every 

 one would acknowledge as good species. " 



Var. HINDSII (Sowb.) Reeve. PL 62, figs. 99, 100; pi. 63, fig. 

 57. 



Shell elongated and much depressed, (the angle of divergence 

 about 140), the girdle encroaching at the sutures. External color 

 a light or dark shade of olive. Surface smoother than in typical 



