XX POLYPLACOPHORA. 



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

 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 (Schizochitonidfx Dall). 



Tail valve fissured. 



Lorica H. and A. Adams. Schizochiton Gray. 



Aulacochiton (Shuttleworth) Cpr. 



F. PLACIPHOROIDEA (Placophoridce Dall). 

 Tail valve unslit, internally ridged, mucro nearly terminal. 

 Enoplochiton Gray. Fremblya H. Adams. 



Ornithochiton Gray. Euplaciphora Shuttleworth. 



Placiphora Gray. Guildingia Cpr. 



G. MOPALOIDEA (Mopaliidce Dall). 

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

 Mopalia Gray. Macandrellus Cpr. 



Placiphorella Cpr. Stectoplax Cpr. 



Katherina Gray. Notoplax H. Adams. 



Acanthochiton (Leach) Herrm. 



H. CRYPTOIDEA (Amiculidce Dall). 



With double sutural laminae. 



Cryptoconchus Blainville. Chlamydochiton Dall. 



Amicula Gray. Cryptochiton Gray & Middendorf. 



Amicula s. s. Dall. 



I. CHITONELLOIDEA ( Cryptoplacidce Dall). 



Tail plate funnel-shaped. Laminse thrown forward. 



Chitonellus Blainville. Choneplax Cpr. 



Cryptoplax 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 Eegu. 



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- 



