318 ATRYPID^. 



in p. ? brevirostris (Devonian, Newton) the dorsal valve has a 

 long trough-like process supported by a single low septum. 



GYPiDiA, Dalman, 1828. P. conchidium, Dalm. Gotland. TJ. 

 Silurian. 



PENTAMERELLA, Hall, 186Y. Ovatelj rounded, with a sinus on 

 the ventral and a mesial fold on the dorsal valve ; dorsal valve 

 with the crura conjoined so as to form a separate trough -shaped 

 cavit}', which unites with the inner surface of the valve ; a narrow 

 area on each side of the fissure, and a flattened space or false 

 area along the cardinal margin of the valve. Devonian ; N. Am. 

 P. arata^ Conr. 



GYPiDULA, Hall, 1861. Short, gibbous or ventricose ; ventral 

 valve much the larger, with or without mesial fold, a large fissure, 

 and elongate, much incurved, trough-shaped pit ; dorsal valve 

 depressed in front ; an area on both valves, that of the ventral 

 valve striate, as in Spirifera ; lamellae of dorsal valve separate 

 and diverging. Devonian ; North America. P. occidentalism 

 Hall. 



ANASTROPHTA, Hall, 1867. (Brachymerus, Shaler, 1865, preoc. 

 Coleopt.) Rotund or gibbous, with the valves, as in ordinary 

 Pentamerus, reversed ; ventral valve smaller, gibbous in its 

 upper part, depressed or sinuate below, with the Y-shaped 

 pit sessile for nearly its entire length ; a small flattened space 

 on each side of the fissure ; dorsal valve ventricose, larger than 

 the ventral, with prominent umbo ; hinge-plate extended in 

 gradually couA^erging vertical lamellae, which are joined to the 

 shell throughout their length, whilst the crura are extended into 

 the cavitj^ in thin, free lamellse. Silurian ; North America. P. 

 Verneuili, Hall. 



AMPHiGENiA, Hall, 186*7. Elongate, not lobed, ventral valve 

 with connected dental lamellae, forming a trough supported on a 

 septum; dorsal valve with free crura; no area; shell-structure 

 punctate. P. elongata, Vanuxem. Devon. ; U. S. 



? CLORiNDA, Barrande. Silui'ian ; Bohemia. 



Family ATRTPID^. 



Shell fibrous ; beak curved ; hinge-margin curved, with strong 

 teeth, without area ; dorsal valve with two spirally coiled lamellse, 

 the points directed towards the middle of the opposite valve. 



Atrypa, Dalman, 1828. 



Syn. — Cleiothj-ris, Phillips, 1841. Spirigeriiia, d'Orb., 184T. 



Disfr. — Fossil, 21 sp. Lower Silurian — Trias; America (Wel- 

 lington Channel! Falkland Islands), Europe, Thibet. A. reticu- 

 laris, Linn, (cxxxvi, 88-91). 



Shell impunctate ; oval, usually plaited and ornamented with 

 squamose lines of grov/th ; dorsal valve gibbose ; ventral 



