ORDER ii POLYPLACOPHORA 435 



}r>i,tinrthrum, Cpr. Both anterior and posterior vahvs with smooth tm.-lit 

 insertion plates, the others lacking them; girdle downy, with small sutural pores. 

 Recent. 



Choriplax, Pik Valves partly immersed in the minutely granulose girdle, all 

 with thin, smooth insertion plates. Recvnt. 



Sub-Order B. MESOPLACOPHORA. Pilsbry. 



Insertion plates developed, slit, not vcrtn-nl!,/ grooved r ji-ti,i,itcd outside. 



Family 1. Ischnochitonidae. Pilsbry. 



Valves having the inner layer well covered by the outer. Surface of intermediate 

 valves divided into lateral and central areas by a diagonal rib (often indistinct), extending 

 from the beak to each anterior 'outer angle of tegumentum ; or when this is not clearly the 

 case, the posterior valve has a crescentic series of well-developed teeth ; all valves with 

 slits. Eocene to Recent. 



Two sub-divisions of this family are recognised, according as the anterior and side 

 slits correspond in position with ribs on the external surface or not Among the 

 genera included under the first head (Callistoplacinae) may be mentioned the follow- 

 ing : Callistochiton, Nuttallina, and Callistoplax, Carpenter ; Craspedochiton, Shuttle- 

 worth ; and Ceratozona, DalL Representatives of the second sub-family (Ischnochitoninae) 

 are as follows : Schizoplax, Dall ; Tonicella, Trachydermon, and Dinoplax, Carpenter ; 

 Callochiton and Ischnochiton, Gray ; Chaetopleura, Shuttleworth. 



Family 2. Mopaliidae. Pilsbry. 



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

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

 ivith 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 Plaxiphora, 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 by the union of the lateral and pleural 

 areas of normal Chitons. Insertion teeth sharp, rarely smooth ; posterior valve either slit 

 like the head-valve, or having a posterior sinus; head-valve USIK&I trith Jive slits, 

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



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

 Carpenter ; Katharina and Amicula, Gray ; Cryptochiton, Midd. and Gray. 



Family 4. Cryptoplacidae. DalL 



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

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

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

 insertion plate continuous behind; girdle very thick and wide. 



This is a highly specialised branch of a low group of Chitons, unknown in tli<- 

 fossil state. Cryptoplax, Blainville (Chitonellus, Lam.), and Choneplax, Carpenter, are 

 examples. 



