302 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



II. Irregular Chitons. 



Tail-plate with a sinus behind. 



The Regular Chitons comprise — 



' A. Lrptoklca. 



Destitute of teeth or slit insertion-plates. 



B. Ischnoidea. 



Insertion-plates slit, sharp, thin ; protected by eaves. 



This contains hy for the largest number of species, and might be con- 

 sidered t^^iical ; but the organization is not as comjtlete in all points as 

 in the next group. 



C. Lopliyroidca. 



Insertion-plates broad, pectinated ; jugular sinus broad, dentate. 



D. Acanthoidca. 



Insertion-plates sharp, grooved externally, eaves furrowed beneath, mucro 

 posteriorly extended. • 



This forms a passage toward II. 

 The Irregular Chitons comprise — 



E. Schizoidea. 



Mantle and tail-plate both slit, behind. 



F. Placiplioroidca. 



Posterior insertion-plates only represented by a pair of swollen ribs. Mantle 

 hairy. 



G. Mopalohlca. 



Tail-plate with one slit on each side and waved behind. 

 H. Cryptoidea. 



Valves covered, or nearly so, with posterior as well as anterior sutural laminae. 

 I. Chitonello'idea. 



Tail-irlato twisted into a funnel, body anteriorly extended. 



Of these groups, Dr. Carpenter says : " I have purposely abstained 

 fi'om giving the usual terminations in idw and irifc because I am not sure 

 that the groups here proposed are entitled to rank even as subfamilies." 



It seems to the writer that these groups are by no means of equal value, 

 and that the Eegidar Chitons might well be reduced to two : Leptoids, 

 and the remainder combined into one group; while the second section 

 might be assorted into Schizoids (including F and G), Cryptoids, and 

 Cliitonelloids. 



The opinions of Dr. Carpenter, the result of years of study, and an 

 examination of all the principal collections of these animals in the world, 

 are, however, not to be hghtly set aside. 



The Chitons of Alaska forming the principal subject of this report, 

 together with notes on allied or extra-limital forms, are now in order. 

 For the use of the figures illustrating this article, and many other favors 

 and facilities for study, I am indebted to the Smithsonian Institution, in 

 charge of Prof. S. F. Baird. 



