512 



MOLLUSCA 



PHYLUM VI 



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

 elevated terminally. Silurian. 



Priscochiton Billings. Similar in the non-sinuous liead-valve, lait beaks of the 

 valves greatly produced backwards. Ordovician. 



Grijphochiton Gray (Fig. 841). 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 Carp. {Anthracochiton Rochebr.). Elon- 



gated, the valves strongly beaked and laterally excavated ; 

 tail -valve with de2)ressed post -median mucro and no 

 posterior sinus, anterior valve with the front margin 

 elevated, sutui-al laminae large. Carboniferous. 



Gymatochiton 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. 

 Probolaeiim Carp. Elongated, elevated ; valves very 

 strongly beaked, the pleura projecting beyond the jugal 

 tract ; anterior valve sinuate in front, posterior valve 

 unknown. Devonian. 



Chonechiton Carp. Median valves as in Gryphochitoyi ; 

 posterior valve with the mucro 

 thrown backward, as in Grypto'plax. 

 Carboniferous. 

 Somewhat like the Recent Lorica, Init 



Fig. S41. 



Gryphochiton ju-iscHS (Munst.). 

 Carboniferous ; Tournay, BRlgiuiii. 

 A, Anterior and three intermediate 

 valves. B, Posterior valve, ventral 

 and dorsal aspects. Vi- 



Loricites Carp, 

 without insertion plates. Carboniferous. 



Family 2. Lepidopleuridae Pilsbry. 



Fig. 842. 



Lepidopleurus virgifer 

 (Sandb.). Miocene ; Wald- 



Insertion plates absent, or present and unslit ; end-valves biickeiheim, near Creuz- 



... r, . 7j7_f ; 1.7 nacli, Germany. A median 



With the terminal margins never elevated ; form oval or oolong, ^ud a posterior valve, '•^/i. 



Tertiary and Recent. 



Leindo'pleurus Risso {Lcptochiton 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. 

 Recent. 



Ghoriplax Pils. Valves partly immersed in the minutely granulose 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 ivell covered by the outer. Surface of intermediate 

 valves divided into lateral and central areas by a diagonal rib {often indistinct), extending 

 from the beak to each anterior aider angle of tegmentum; or ivhen this is not clearly the 

 case, the posterior valve has a crescentic series of wdl-developed teeth ; all valves with 

 slits. Eocene to Recent. 



