Vol. VII] BERRY— WEST AMERICAN CHITONS 241 



19. Mopalia muscosa (Gould) 



This species would appear to be excessively rare, only a 

 single specimen having been taken inside Cape Lookout, Dall 

 Island, and none at all at the other localities. The specimen is 

 not a very large one, but seems typical. 



20. Mopalia imporcata Carpenter 



Dredged in 15-25 fathoms at Forrester Island, and appar- 

 ently the common Mopalia in all the shallower depths. The 

 specimens are very uniform in size and general appearance, but 

 exhibit some variation in the minor details of sculpture. 



21. Mopalia sinuata Carpenter 



A few specimens were taken with the last species in 15-20 

 fathoms at Forrester Island, but not very frequently. 



22. Placiphorella velata Carpenter 



In Mr. Willett's experience this is another of the notably rare 

 chitons. Only two specimens were taken, both at low tide on 

 Forrester Island in 1915. 



23. Placiphorella rufa, new species 



More common than P. velata at Forrester Island, though 

 always dredged, is the very interesting Placiphorella now to be 

 described. 



Diagnosis: Shell of moderate size, broadly oval in outline, 

 depressed. Dorsal ridge only moderately elevated, the side 

 slopes little convex. 



Anterior valve crescentic, showing strong, irregular lines of 

 growth and occasional weak traces of radial grooves. Median 

 valves with the lateral areas raised into diagonal and sutural 

 ribs having a groove-like depression between; otherwise un- 

 sculptured except for the strong, uneven lines of growth, 

 especially prominent where they traverse the ribs; not beaked. 

 Posterior valve small, only about half as wide as the anterior 

 valve; sculptured by two strong, oblique ribs converging to the 

 depressed, distinctly posterior, but not marginal, mucro. 



