512 



MOLLUSCA 



PHYLUM VI 



Helminthochiton Salter. Valves tliin, mucro subcentral, low; end-valves not 

 elevated terniinally. Silurian. 



Priscochiton Billings. Similar in the non-sinnous liead-valve, biit beaks of tlie 

 valves greatly produced backwards. Ordovician. 



Gryphochiton Gray (Fig. 841). Elongated, with small beaks and very small sntural 

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

 decurved mucro behind the middle, Carboniferous. 



Pterochiton Carp. (Änthracochiton 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. 



Gymatochiton Dali (Protalochiton Rochebr.). Oval, 

 elevated and granulär, the valves short and wide, with 

 small, low sutural laminae and distinct lateral areas ; 

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

 Proholaeum Carp. Elongated, elevated ; valves very 

 strongly beaked, the pleura projecting beyond the jugal 

 tract ; anterior valve sinuate in front, posterior valve 

 unknown. Devonian. 



Ghonechiton Carp. Median valves as in Gryphochiton ; 



Fig. 841. 



Gryphochiton priscus (Münst.). 

 Carboniferous ; Tournay, Belgium. 

 J, Anterior and threeintermediate posterior valve with the mucro 

 valves. B, Posterior valve, ventral ^ , • y^ , 7 



and dorsal aspects. Vi- thrown backward, as m Cryptoptax. 



Carboniferous. 



Loricites Carp. Somewhat like the Recent Lorica, Init 



without Insertion plates. Carboniferous. 



Family 2. Lepidopleuridae Pilsbry. 



Fig. 842. 



Lepidopleurus 

 (Sandb.). Miocene ; Wald- 

 böckelheim, near Creuz- 

 nach, Germany. A median 

 and a posterior valve. '-^/i. 



Insertion plates ahsent, or present and unslit ; end-valves 

 with the terminal margins never elevated; form oval or oblong. 

 Tertiary and Recent. 



Lepidopleurus Risso {Leptochiton Gray) (Fig. 842). Small, oval ; Insertion plates 

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

 Recent. 



Hanleyia Gray. Like the last, except that the anterior valve has an unslit Insertion 

 plate, and the girdle is spiculose. Champlain to Recent. 



Hemiarthrum Carp. Both anterior and posterior valves with smooth unslit 

 Insertion plates, the others lacking them ; girdle downy, with small sutural pores. 

 Rec^int. 



Ghoriplax Pils. Valves partly immersed in the minutely granulöse girdle, all with 

 thin, smooth Insertion plates. Recent. 



Suborder B. MESOPLACOPHORA Pilsbry. 



Insertion plates developed, slit, not vertically grooved or pectinated outside. 



Family 1. Ischnochitonidae Pilsbry. 



Valves having the inner layer well covered hy the outer. Surface of intermediate 

 valves divided into lateral and central areas hy a diagonal rib (pften indistinct), extending 

 from the beah to each anterior outer angle of tegmentum ; 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. 



