POLYPLACOPHORA. Xlll 



translucent minor lateral of varying form ; a major lateral larger 

 than any of the other teeth, with a conspicuous black cusp, which 

 may have from one to four denticles ; two boss-like or thickened 

 uncinal plates of irregular shape ; a twisted spatulate uncinal, and 

 three scale-like or slightly thickened external uncini. With the 

 exception of the spatulate uncinus (which is aborted in a very few 

 species) none of the uncini are much raised above the plane of the 

 odontophore, and none present any characters of importance." 



Dall has figured and described the dentition of species belonging 

 to over half of the genera and subgenera recognized by Carpenter 

 including all of the main types. He concludes that as yet no char- 

 acters affording grounds for even generic distinctions can be found 

 in the teeth, which exhibit a most surprising uniformity throughout 

 the entire group. The dentition of a number of genera not exam- 

 ined by Dr. Dall has been worked out by the writer, but without 

 result as far as the discovery of structures of taxonomic value is 

 concerned. It is evident that while some minor features of the 

 dentition are characteristic of groups founded upon other charac- 

 ters, we need expect little assistance in larger questions of group 

 affinities from the further investigation of this organ. 



The gills are "posterior" or short in the lower Chitons, the 

 Lepidopleuridce, and also in the lowest Ischnochitonidce, and in most 

 genera of the phylum Acanthochitidce Cryptoplacidce, which arose 

 from an early Ischnoid stock. All other Chitons, including the 

 higher Ischnochitonidce, the Mopaliidce and the Chitonidce, have 

 " ambient " or long gills. 



TERATOLOGY. 



The occurrence of six- and seven-valved Chitons has been noted 

 as early as the time of Linnseus. It is likely that the six-valved 

 were artificial fabrications, although a certain number may perhaps 

 be traced to incorrect drawings. Most seven-valved specimens are due 

 to the soldering together of two valves in consequence of some injury. 

 This is not uncommon, a number of cases having come under my 

 observation. Individuals actually seven-valved are known to occur, 

 although they are far from common. The writer has examined many 

 thousand Chitons, but has seen only two normally 7-valved examples. 

 One of these is figured on pi. 64, fig. 67 ; and as may be seen, it is in 

 no respect abnormal save in the want of one central valve, and the 

 consequent lengthening of the others. There is absolutely no trace 



