XX POLYPLACOPHORA. 



***Plates sharp, smooth (A. ischnoidea). 

 Dinoplax Cpr. Nuttallina Cpr. 



Middendorfia Cpr. Arthuria Cpr. 



Beanella Dall. Phacellopleura Guilding. 



Section II. Chitones irregulares (OPSICHITONIA Dall). 

 Tail plate abnormal or with a sinus behind. 

 E. SCHIZOIDEA (Schizochitonidw Dall). 



Tail valve fissured. 



Lorica H. and A. Adams. Schizochiton Gray. 



Aulacochiton (Shuttle worth) Cpr. 



F. PLACIPHOROIDEA (Placophoridce Dall). 

 Tail valve unslit, internally ridged, mucro nearly terminal. 

 Enoploehiton Gray. Fremblya H. Adams. 



Ornithochiton Gray. Euplaeiphora Shuttleworth. 



Placiphora Gray. Guildingia Cpr. 



G. MOPALOIDEA (Mopaliidce Dall). 

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

 Mopalia Gray. Maeandrellus Cpr. 



Placiphorella Cpr. Stectoplax Cpr. 



Katherina Gray. Notoplax H. Adams. 



Acanthochiton (Leach) Herrm. 



H. CRYPTOIDEA (Amiculidce Dall). 



With double sutural laminae. 



Cryptoconclius Blainville. Chlamydochiton Dall. 



Amicula Gray. Cryptochiton Gray & Middendorf. 



Amicula s. s. Dall. 



I. CHITONELLOIDEA ( Cryptoplacidce Dall). 

 Tail plate funnel-shaped. Laminae thrown forward. 

 Chitonellus Blainville. Choneplax Cpr. 



Oryptoplax Gray. Chitoniscus 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- 



