XX POLYPLACOPHORA. 



***Plates sharp, smooth (A. /. 

 l)iimplax Cpr. Xiiff'illui'i Cpr. 



Mi'ldcwlorjia ('|> r - !r. 



Beaiiella Dall. /V//(v//oyA //ra Guild! ng. 



Section II. Chitones irregulares (OPSICHITONIA Dall). 

 Tail plate abnormal or with a sinus behind. 

 E. SCHIZOIDEA (Schizochltonidce Dall). 



Tail valve fissured. 



a H. and A. Adams. ochiton Gray. 



Aulacochiton (Shuttleworth) Cpr. 



F. l*i..\ im<>i:<uin:A (Placophoridce Da\\). 

 Tail valve unslit, internally ridged, mucro nearly terminal. 

 Enoplorhiton Gray. Fremblya H. Adams. 



Ornithnchiton Gray. Euplaciphora Shuttleworth. 



Plncifthora Gray. Guildingia Cpr. 



G. MOPALOIDKA (Mopaliidto Dall). 



Tail valve with posterior sinus and one slit on each side. 

 iii ( ray. Macmidrellus Cpr. 



FlaeqphoreUa Cpr. Stectoplax Cpr. 



Rather inn ( Jray. Notoplax H. Adams. 



thochiton (I^each) Herrm. 



H. CRYPTOIDEA (Amiculidce Dall). 



With double sutural laminae. 



'foconchus Blainville. Chlamydochiton Dall. 



Amicula Gray. Cryptochiton Gray & Middendorf. 



Amicula s. s. Dall. 



I. CHITONELLOIDEA (Cryptoplacidce Dall). 

 Tail plate funnel-shaped. Laminse thrown forward. 

 Chitoncllus Blainville. Choneptax Cpr. 



Cnjptoplax Gray. CMtoniteui Cpr. 



There can be no doubt that Carpenter's classification is a great 

 advance upon that of Gray. It will be noticed that the primary 

 division of the family is into two great groups: (1) Chitons having 

 the head and tail valves similarly articulated with the girdle, and 

 (2) those having them strongly dissimilar. These two great groups 

 were early recognized by Carpenter, and called respectively Regu. 

 lar and Irregular Chitons. 



DALL, in his Report on the Chitons of Alaska (1878), accepts the 

 Carpenterian classification, correcting it in some details, notably in 

 the arrangement of the Amiculoid forms. In the same year he 

 published an extensive and extremely valuable paper upon the den- 



