PROCEEDINGS OF UNITED STATES NATIONAL MUSE DM. 301 



remains to rectify the disorder by the light of present knowledge, a task 

 which may not long be delayed. If some modern authors, who have 

 instituted wholesale changes in nomenclature, had followed a consistent 

 and uniform plan, and not neglected or hurriedly decided on doubtful 

 points, the work of rectification might have been much more simple, 

 though perhaps not less urgently needed. 



A few words may be added in regard to the names given by Midden - 

 dorf. In spite of the opportunities afforded by his study of the Russian 

 Chitons, this distinguished savant seemed to fail to catch the permanent 

 as distinguished from merely individual characters, and his classifica- 

 tion and nomenclature are not borne out by subsequent researches. 

 His chief characters were derived from the dimensions of the soft or 

 coriaceous girdle, dimensions which differ not only in the same species, 

 but in the same individual, respectively, if preserved in spirit (when it 

 may be broad) or dry (when it shrinks to a narrower compass). From 

 this cause it is not surprising to find the same species figuring in both 

 of his chief divisions of Chitons with exposed valves. In the attempt 

 to utilize this impracticable classification, and unwilling to admit that 

 the Chitonidce contain more than one genus, he adopted a singular 

 nomenclature, in which the genus was divided, into a great number of 

 sections, subsections, sub-subsections, etc., so that his work can hardly be 

 classed as binomial in the Linnean sense. Fortunately, without excep- 

 tion, the groups indicated had previously been properly named by Gray, 

 and only by courtesy can the genus Cryptocltiton, on which his industri- 

 ous research was largely expended, be assigned to him as authority, 

 since it was denominated by the same name by Dr. Gray but a short 

 time previously, the researches of each being unknown to the other. 



To Blainville, in 1810, is due the credit of first recognizing the anom- 

 alous characters of the Chitonidce, and their separation as an independent 

 group from other gasteropods. While the value of a class in view of 

 later researches may be held to be too high, yet few will be disposed to 

 deny them the ordinal value assigned by Gray in 1825. The name is 

 preferably spelled Polyplaeipliora, though numerous other forms have 

 been used. 



The order Polypktciphora can with certainty be asserted to contain but 

 one family, so far as our present knowledge is concerned. No groups of 

 subfamily value have yet been recognized, and it is a question whether 

 any exist. It would be out of place here to attempt any resume of the 

 various systems of classification proposed by authors who have written 

 on Chitons, as that proposed by Dr. Carpenter has solely been followed, 

 and the process would occupy too much space. 



Dr. Carpenter's arrangement is founded upon the plan of structure in 

 the valves, the extent of the branchiae, and the ornamentation or char- 

 acter of the girdle and its covering, lie divides the Polyplaciphora into 

 two great divisions : 



I. Regular Chitons. 



Head and tail plates of similar character. 



