434 



MOLLUSCA 



SUB-KINGDOM VI 



valves, fish scales, and other fragments. The recently described Duslia 

 insignis, Jahn, is apparently a Crustacean ; certainly not a member of the 

 Polyplacophora. 



Three sub -orders are recognised, according as the insertion plates are 

 absent, or if present, unslit (Eoplacophora) developed, smooth, and slit into 

 teeth (Mesoplacophora) ; or both slit and pectinated (Teleoplacophora). 



I 



Sub-Order A. EOPLACOPHORA. Pilsbry. 



Polyplacophora with the tegumentum coextensive with the articulamentum, or ivith the 

 latter projecting in smooth, unslit insertion plates ; gills posterior. 



Family 1. G-ryphochitonidae. Pilsbry. 



Insertion plates absent, sutural laminae small; one or both end-valves ivith the 

 terminal margins elevated ; form elongated and narrow. Palaeozoic. 



Helminthochiton, Salter. Valves thin, mucro sub -central, low; end -valves not 

 elevated terminally. Silurian. 



Priscochiton, Billings. Similar in the non-sinuous head-valve, but beaks of the 

 valves greatly produced backwards. Orclovician. 



Gryphochiton, Gray (Fig. 792). Elongated, with small beaks and very small sutural 

 laminae; terminal margins of end-valves strongly elevated; tail-valve with low, 

 decurved mucro behind the middle. Carboniferous. 



Pterochiton, Cpr. (Anthracochiton, Eochebr.). Elon- 

 gated, the valves strongly beaked and laterally excavated ; 

 tail -valve with depressed post -median mucro and no 

 posterior sinus, anterior valve with the front margin 

 elevated, sutural laminae large. Carboniferous. 



Cymatochiton, Dall (Protalochiton, Eochebr.). Oval, 

 elevated and granular, the valves short and wide, with 

 small, low sutural laminae and distinct lateral areas ; 

 mucro of posterior valve post-median, elevated. Permian. 

 Probolaeum, Cpr. Elongated, elevated ; valves very 

 strongly beaked, the pleura projecting beyond the jugal 

 tract ; anterior valve sinuate in front, posterior valve 

 unknown. Devonian. 



Chonechiton, Cpr. Median valves as in Gryphochiton ; 



with the mucro 

 thrown backward, as in Gryptoplax. 

 Carboniferous. 



Somewhat like the recent Lorica, but 

 without insertion plates. Carboniferous. 



Family 2. Lepidopleuridae. Pilsbry. 



T .. Sandb. ^Miocene ; Wald- 



insemon plates absent, or present and unslit ; end-valves bockeiheim, near Creuz- 



toith the terminal margins never elevated; form oval or oblong, "nda Serior'vah-e! 16 ^" 

 Tertiary and Eecent. 



Lepidopleurus, Eisso (Leptochiton, Gray), (Fig. 793). Small, oval ; insertion plates 

 entirely absent, sutural laminae small ; girdle minutely scaly or chaffy. Eocene to 

 Eecent. 



FIG. 792. 



Gryphochiton priscus, Munst. 

 Carboniferous ; Tournay, Belgium. 

 A, Anterior and three intermediate , 

 valves. B, Posterior valve, ventral posterior valve 

 and dorsal as 



Hanleya, Gray. 

 plate, and 



Like the last, except that the anterior valve has an unslit insertion 

 i s spiculose. Champlain to Eecent. 



