CHITON. 151 



valves, a meaning also favored by the etymology ; but in P. Z. S. 

 1847 he places Gymnoplax under Chiton s. s. as a synonym. In 

 any case, the name Gymnoplax cannot be used, as it has never been 

 diagnosed except in the general way quoted above, and no species 

 has ever been expressly designated as belonging to it. The name 

 has been used by Gray only in the instances above mentioned and 

 was, as we have seen, considered a synonym by him in his later 

 writings ; but it should be noted that the irrepressible Rochebrune, 

 still uses Gymnoplax in a generic sense, in order presumably, to 

 disguise his species ; an unnecessary precaution, for his diagnoses of 

 Chitons generally defy indentification of either genus or species. 



In conclusion : it is obvious that the name Chiton must be retained 

 for a group typified by one of Linne's original species ; and since the 

 tuberculatus is the only one of the original list which has been or 

 can be identified, that species becomes the type of the genus. No 

 other position on this question is reasonable or tenable. 



The most natural primary division of Chiton is into two groups ; 

 one to include all American and some Old World species, in which 

 the mucro is anterior and the scales smooth, the other to include Old 

 World species having the mucro subcentral and the scales striated. 

 As this division is based upon characters not always easy to see, the 

 following division into sections is more convenient. 



Section CHITON (restricted.) 



Median valves having a single slit in each insertion-plate; sinus 

 generally denticulate ; scales closely imbricating. 



Section RADSIA Gray. 



Median valves having two or more slits in each insertion-plate. 

 Section SCLEROCHITON Cpr. 



Median valves having a single slit in each insertion-plate ; teeth 

 of tail-valve tending forward ; sinus smooth ; scales of girdle 

 separated. 



Section CHITON s. sir. 



Artificial key to species of Chiton. 



A. LATERAL AREAS RADIALLY SCULPTURED. 



a. Central areas having longitudinal riblets. 



b. Sides and ridge of central areas both sculptured. 



