394 



MOLLUSCOIDEA 



PHYLUM V 



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

 Russia. 



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

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

 Russia. 



Subfamily B. Parastrophiinae, novum. 

 Plicated Porambonitidae. 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. 



Anastrophia Hall (Brachymerus Shaler, noii 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 Porambonitidae. 

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

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

 ventral valve, Fig. 7) ; h, Dental lamellae ; c, Crura ; ,S', Septum-like supports, Fig. C. 



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. 



