DESCRIPTIONS OF NEW SPECIES OF CHITONS FROM 

 THE PACIFIC COAST OF AMERICA. 



By Willia:m Healey Dall, 



Honorary Curator of MoUusks, United States National Museum. 



Ill reviewing the West American Mollusca for the purpose of 

 completing a checklist of Pacific coast Gastropoda, a large number 

 of Chitons which appeared to be undescribed were found in the col- 

 lection. Part of these were minute, and in the fear that with more 

 thorough study than it was in my power to give at this time they 

 might prove to be young of species already known, and in view of 

 the well-known variability of many groups of these animals it was 

 thought best to omit them from consideration at present. The others 

 are now described. I have intentionally referred them to the larger 

 groups on account of the difficulty in determining the value of sub- 

 divisions which have been proposed by recent writers. As the 

 specimens were all dry it has been impossible in a number of cases 

 to determine the extent of the gill rows, but this has been stated 

 whenever practicable. The dried specimens, being in various stages 

 of contraction, it will be understood that the measurements given 

 merely approximate those of the living animals. 



There is little doubt that the west coast of America is the richest 

 Chiton region in the world, that many new forms remain to be 

 discovered and that a satisfactory classification will in time be 

 worked out. The present paper is a small contribution toward that 

 end. 



LEPIDOPLEURUS (LEPTOCHITON) AMBUSTUS, new species. 



Chiton with a rather high but rounded back, the valves reddish- 

 brown with numerous blackish flecks, giving the interspaces a some- 

 what vermicular aspect; anterior valve semicircular, simple; pos- 

 terior valve smaller, shorter, with a subcentral prominent mucro be- 

 hind which the surface is concavely excavated; intermediate valves 

 subequal, dorsally rounded with hardly any trace of lateral areas; 

 sculpture of the valves nearly uniform fine equal grooves, axially on 

 the dorsal area, oblique on the lateral areas and cut by still finer 

 transverse striae into microscopic granulations; the distal ends of 

 the middle valves in the adult shell with concentric undulations ; in- 



Proceedings U. S. National Museum, Vol. 55— No. 2283, 



499 



