THE AMPHINEURA 53 
V. SYSTEMATIC REVIEW OF THE SUB-ORDERS AND FAMILIES OF 
THE POLYPLACOPHORA. 
Sus-OrpDER 1. EorLacopHora, Pilsbry. 
Tegmentum coextensive with articulamentum, or the latter projecting 
in smooth, unslit plates. 
Famity 1. LerprporLeuripar, Pilsbry. Articulamentum without 
insertion plates or with unslit plates ; terminal margins of end valves 
never elevated; form oval or oblong, Genera— Lepidopleurus, Risso 
(= Leptochiton, Gray) (Fig. 28, A). Without insertion plates, sutural 
laminae small; girdle minutely scaly or chaffy. JL. cancellatus, Sow. 
North Atlantic and Mediterranean ; various abyssal species. Hanleya, Gray. 
The anterior valve with an unslit insertion plate. H. hanleyi, Bean. North 
Atlantic. Hemiarthrum, Carpenter. Both anterior and posterior valves 
with smooth, unslit insertion plates; girdle downy. Microplax, Adams 
and Angas. 
To this group belongs the extinct family of GryYPTocHITONIDAE, 
Pilsbry, together with various narrow and elongated Palaeozoic genera, 
whose one or two end valves have the terminal margins elevated. 
SuB-OrDER 2. MesopnuacopHora, Pilsbry. 
Insertion plates well developed and slit. 
Famity 2. IscHNocurronipak, Dall. All the valves with slits, and 
the inner layer well covered by the outer. Sus-Famity 1. IscHno- 
CHITONINAE. No shell-eyes ; sutural laminae separated ; the slits in the 
valves 1 to 7 do not correspond with the ribs of the tegmentum. 
Genera—TIschnochiton, Gray. Smooth girdle. Trachydermon, Carpenter 
(with the sub-genera: Tonicella, Carpenter, and Boreochiton, Sars). Girdle, 
with small squamous spicula. 7. cinereus, Linnaeus (Fig. 28, B) (= T. 
marginatus, Pennant). North Atlantic. Chaetoplewra, Shuttleworth. 
Hairy girdle. Stenoplax,Carpenter. Stenoradsia,Carpenter. SuB-FamIzy 2. 
CALLOCHITONINAE. With shell-eyes and united sutural laminae. Genus— 
Callochiton, Gray. C. laevis, Pennant. North Atlantic and Mediterranean. 
Sub-Faminy 3. CALLISTOPLACINAE. No shell-eyes; the slits in the 
valves 1 to 7 corresponding with the ribs of the tegmentum. Genera— 
Callistochiton, Carpenter (viviparous). Nuttalochiton, Plate. 
Famity 3. MopauipaAr, Dall. Each intermediate valve with a 
single slit; girdle hairy. Genera— Mopalia, Gray.  Placiphorella, 
Carpenter (Fig. 32). Plaxiphora, Gray. Placophoropsis, Pilsbry. 
Faminy 4. ACANTHOCHITONIDAE, Pilsbry. Valves immersed in the 
girdle, and with small tegmentum. Genera— Acanthochiton, Leach 
(= Cryptoconchus, Guilding). Girdle with bundles of spicula. A. fascicu- 
laris, L. North Atlantic and Mediterranean. Spongiochiton, Carpenter. 
Katharina, Gray. Amicula, Gray. Cryptochiton, Middendorf. Valves 
quite concealed ; no tegmentum. CC. stelleri, Middendorf. Arctic. 
Faminy 5. Crypropnactpa®, Dall. Vermiform, with thick girdle 
and small valves ; insertion and sutural plates strongly drawn forward, 
