THE AMPHINEURA 53 



V. SYSTEMATIC REVIEW OF THE SUB-ORDERS AND FAMILIES OF 



THE POLYPLACOPHORA. 

 SUB-ORDER 1. EOPLACOPHORA, Pilsbry. 



Tegmentum coextensive with articulamentum, or the latter projecting 

 in smooth, unslit plates. 



FAMILY 1. LEPIDOPLEURIDAE, Pilsbry. Articulamentum without 

 insertion plates or with unslit plates ; terminal margins of end valve& 

 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. L. 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 An gas. 



To this group belongs the extinct family of GRYPTOCHITONIDAE, 

 Pilsbry, together with various narrow and elongated Palaeozoic genera, 

 whose one or two end valves have the terminal margins elevated. 



SUB-ORDER 2. MESOPLACOPHORA, Pilsbry. 



Insertion plates well developed and slit. 



FAMILY 2. ISCHNOCHITONIDAE, Dall. All the valves with slits, and 

 the inner layer well covered by the outer. SUB-FAMILY 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 Ischnochiton, Gray. Smooth girdle. Trachydermon, Carpenter 

 (with the sub-genera : Tonicella, Carpenter, and Boreochiton, Sars). Girdle, 

 with small squamous spicula. T. cinereus, Linnaeus (Fig. 28, B) ( = T. 

 marginatus, Pennant). North Atlantic. Chaetopleura, Shuttleworth. 

 Hairy girdle. Stenoplax, Carpenter. Stenoradsia, Carpenter. SUB-FAMILY 2. 

 CALLOCHITONINAE. With shell-eyes and united sutural laminae. Genus 

 Callochiton, Gray. C. laevis, Pennant. North Atlantic and Mediterranean. 

 SUB- FAMILY 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. 



FAMILY 3. MOPALIIDAE, Dall. Each intermediate valve with a 

 single slit ; girdle hairy. Genera Mopalia, Gray. Placiphorella, 

 Carpenter (Fig. 32). Plaxiphora, Gray. Placophoropsis, Pilsbry. 



FAMILY 4. ACANTHOCHITONIDAE, Pilsbry. Valves immersed in the 

 girdle, and with small tegmentum. Genera Acanthochiton, Leach 

 ( = Cryptoconchus, Guilding). Girdle with bundles of spicula. A. fasricu- 

 laris, L. North Atlantic and Mediterranean. Spongiochiton, Carpenter. 

 Katharina, Gray. Amicula, Gray. Cryptochiton, Middendorf. Valves 

 quite concealed ; no tegmentum. C. stelleri, Middendorf. Arctic. 



FAMILY 5. CRYPTOPLACIDAE, Dall. Vermiform, with thick girdle 

 and small valves ; insertion and sutural plates strongly drawn forward, 



