ORDER II POLY PLACOPHORA 435 
Hemiarthrum, Cpr. Both anterior and posterior valves with smooth unslit 
insertion plates, the others lacking them; girdle downy, with small sutural pores. 
Recent. 
Choriplaz, Pils. Valves partly immersed in the minutely granulose girdle, all 
with thin, smooth insertion plates. Recent. 
Sub-Order B. MESOPLACOPHORA. Pilsbry. 
Insertion plates developed, slit, not vertically grooved or pectinated 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 tequmentum ; 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 Dinoplaz, Carpenter ; 
Callochiton and Ischnochiton, Gray ; Chaetoplewra, 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 tt 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 Plaxiphora, Gray ; 
Placiphorella, Carpenter ; and Placophoropsis, Pilsbry. 
Family 3. Acanthochitidae.  Pilsbry. 
Valves more or less immersed in the smooth or hairy girdle, the teymentum 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 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 ; 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, the 
others with 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 the 
fossil state. Cryptoplax, Blainville (Chitonellus, Lam.), and Choneplax, Carpenter, are 
examples. 
