CLASs III AMPHINEURA 513 



Two subdivisions of this family are recognised, according as tlie anterior and side 

 slits correspond in position witli ribs on the extenial surface or not. Among the 

 genera included under tlie first section {Callistoplacinae) may be mentioned the follow- 

 ing : — CallistocJiiton, Nuttalina, and Callistoplax Carpenter ; Graspedochiton Shuttle- 

 worth ; and Geratozona Dali. Representatives of the second subfamily {Ischnochitoninae) 

 are as foUows : — Schizoplax Dali ; Tonicella, Trachydermon, and Dinoplax Carpenter ; 

 Gallochiton and Ischnochiton Gray ; Ghaetopleura Shuttleworth. 



Family 2. Mopaliidae Pilsbry. 



Valves externally divided tijpically into central and lateral areas, the posterior valve 

 with a sinus hehind, one or two slits pn each side of it or none ; intermediate valves each 

 with a Single slit ; teeth smooth, sharp, often with thickened edges on the outside ; girdle 

 more or less hairy. Pleistocene and Recent. 



This family comprises the following genera : — Mopalia and Plaxiyhora Gray 

 Placiphorella Carpenter ; and Placophoropsis Pilsbry. 



Family 3. Acanthochitidae Pilsbry. 



Valves more or less immersed in the smooth or hairy girdle, the tegmentum therefore 

 much smaller than the articulamentum ; the exposed surface divided into a narrow dorsal 

 and wide latero-pleural areas, the latter formed hy the union of the lateral and pleural 

 areas of typical Ghitons. Insertion teeth sharp, rarely smooth; posterior valve either 

 slit like the head-valve, or having a posterior sinus ; head^valve usually with five slits, 

 intermediate valves singly slit Body never vermiform. Pliocene to Recent. 



The following representatives are to be cited : — Acanthochites Risso ; Spongiochiton 

 Carpenter ; Katharina and Amicula Gray ; Gryptochiton Midd. and Gray. 



Family 4. Oryptoplacidae DalL 



Elongated or vermiform Ghitons with small valves; insertion and sutural plates 

 strongly drawn forward, sharp and smooth ; the anterior valve with three to flve slits, the 

 others with one slit on each side, or none; tail-valve having the mucro far posterior, 

 insertion plate continuous hehind ; girdle very thick and wide. 



This is a highly specialised branch of a low groiip of Chitons, unknown in the 

 fossil State. Gryptoplax Blainville {Ghitonellus Lam.), and Ghoneplax Carpenter, are 

 examples. 



Suborder C. TELEOPLACOPHORA Pilsbry. 



All valves, or the first seven, with insertion plates cut into teeth hy slits ; the teeth 

 sharply sculptured (^ectinated) outside hy fine vertical grooves. 



Family 1. Ohitonidae Pilsbry. 

 Gharacters those of the suhorder. Tertiary and Recent. 



The family is illustrated by the following genera, of which only the first two occur in 

 the fossil condition : — Ghiton Linne ; Trachyodon Dali ; Eudoxochiton Shuttleworth ; 

 Tonicia, Schizochiton, Enoplochiton and Onithochiton Gray ; Acanthopleura Guilding ; 

 Lorica Adams ; Loricella and Liolophura Pilsbry. 



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