KEPOET ON THE POLYPLACOPHORA. 47 



rectum. Respiration is effected in Neomenia by about thirty ciliated folds in the rectum, 

 in Chcetoderma the gills are paired, the anal opening lying between them. The few gills 

 situated on each side of the anus in the Leptoidea is but a slight advance, and from this 

 condition there is a complete series to the long row of closely-set gills of the most 

 specialised Chitons, other details of structure tending to prove that the extent of the 

 gill-row is one indication of specialisation. In Cryptoplax the gills extend to about one- 

 third to one-half of the length of the foot, and thus constitute a distinct advance upon 

 Leptochiton. 



On the whole, then, it would appear that the genus Cryptoplax is a highly specialised 

 branch of a low group of Chitons, but there are not yet sufficient data to indicate the exaet 

 relationship of the genus. The limited Australasian distribution of the genus, so far 

 as that is of any value, supports this conclusion. Carpenter, Dall, and Rochebrune 

 deny that any fossil representatives of the genus are known. 



