394 



MOLLUSCOIDEA 



PHYLUM V 



areas in both valves. Shells thick, variously pitted externally. Ordovician ; 

 Eussia. 



Noetlingia Hall and Clarke. Exterior like Porambonites, but with long, 

 straight hinge-line, prominent cardinal areas, andperforate beaks. Ordovician; 

 Russia. 



Subfamily B. Parastrophiinae, novum. 



PUcated Poramhonitidae. Ordovician to Lower Devonian. 



Camarella Billings. Small, smooth shells with a few low plications and 

 without cardinal areas. Dorsal valve at maturity most convex. Spondylium 

 well defined. Cruralium very small, supported by a long septum. Ordovician ; 

 North America and (?) England. 



Parastrophia Hall and Clarke. Much like Camarella, but with a moderately 

 long, straight cardinal line, and no cardinal area. Dorsal umbo conspicuous, 

 projecting beyond that of the ventral valve. Ordovician and Silurian ; North 

 America and England. 



Änastrophia Hall {Brachymerus Shaler, non Dej.). Much like Parastrophia, 

 but with the dorsal umbo more prominent and the valves with numerous 

 sharp plications extending to the beaks. Silurian and Lower Devonian; 

 North America, England and Gotland. 



(?) Lycophoria Lahusen. Russia and Scandinavia. 



; Family 4. Pentameridae M'Coy. 



Terminal, usually rostrate Pentamerids, developed out of Poramhonitidae. 

 Spondylia and cruralia well developed. Sometimes there is a concave plate in the 

 ventral delthyrium that is interpreted as the "deltidium ; if so, it is a special develop- 

 ment due to the highly incurved beaks of both valves in the decidedly convex species. 

 Silurian to Permian. 



Conchidium Linn. {Gypidia Dalman ; Antirhynchonella Quenstedt ; Zdimir 

 Barrande) (Figs. 587 ; 588, D). Shell strongly inequivalve, biconvex, with 



Fig. 587. 

 Conchidium Uloculare Linn. Silurian ; Gotland. A, Shell of the natural size. B, Beak showing concave 

 deltidium. C, D, Interior of dorsal and ventral valves, respectively. x, Spondylium ; S, Median septum of 

 ventral valve, Fig. D ; b, Dental lamellae ; c, Crura ; S, Septum-like supports, Fig. Ü. 



n 



highly arched ventral valve. Surface numerously plicated. Spondylium narrow 

 and deep, and supported by a high vertical septum of variable length. A 

 small cardinal process present. Silurian and Devonian ; distribution general. 



