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 (Hoplacophora) ; developed, smooth, and slit into 
teeth (Mesoplacophora) ; or both slit and pectinated (Teleoplacophora). 
Sub-Order A. EOPLACOPHORA. Pilsbry. 
Polyplacophora with the tegqumentum coextensive with the articulamentum, or with the 
latter projecting in smooth, unslit insertion plates ; gills posterior. 
Family 1. Gryphochitonidae. Pilsbry. 
Insertion plates absent, sutural laminae small; one or both end-valves with 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. Ordovician. 
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, Rochebr.). 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, Rochebr.). 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 

Fia. 792. : 
|e es ; » unknown. Devonian. 
rryphochiton priscus, Miinst. ‘ > : . 2 
Carboniferous ; Tournay, Belgium. Chonechiton, Cpr. Median valves as in Gryphochiton ; 
A, Anterior and three interinediate Ye ae ee pe ao “5 
valves. B, Posterior valve, ventral POSterior valve with the mucro 
and dorsal aspects. 1/). thrown backward, as in Cryptoplaz. 
Carboniferous. 
Loricites, Cpr. Somewhat like the recent Lorica, but 
without insertion plates. Carboniferous. 

Fic. 793. 
Family 2. Lepidopleuridae. Pilsbry. 
Lepidopleurus  virgifer, 
A ‘ Sandb. Miocene; Wald- 
Insertion plates absent, or present and unslit; end-valves bickelheim, near Creuz- 
aa eis nie : . : nach, Germany. A median 
with the terminal marguns never elevated ; form oval or oblong. anda posterior valve. 2/}. 
Tertiary and Recent. 
Lepidopleurus, Risso (Leptochiton, Gray), (Fig. 793). Small, oval; insertion plates 
entirely absent, sutural laminae small; girdle minutely scaly or chaffy. Eocene to 
Recent. 
Hanleya, Gray. Like the last, except that the anterior valve has an unslit insertion 
plate, and the girdle is spiculose. Champlain to Recent. 
