318 ATRYPIDA. 
in P. ? brevirostris (Devonian, Newton) the dorsal valve has a 
long trough-like process supported by a single low septum. 
GyPipiA, Dalman, 1828. PP. conchidium, Dalm. Gotland. U. 
Silurian. 
PENTAMERELLA, Hall, 1867. Ovately 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 
cavity, 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, 1867. 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; lamelle of dorsal. valve separate 
and diverging. Devonian; North America. P. occidentalis, 
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 V-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 converging vertical lamellae, which are joined to the 
shell throughout their leneth, whilst the crura are extended into 
the cavity in thin, free lamellae. Silurian; North America. /P. 
Vernewili, Hall. 
AMPHIGENTA, Hall, 1867. Elongate, not lobed, ventral valve 
with connected dental lamelle, 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. Silurian; Bohemia. 
Famity ATRYPID A. 
Shell fibrous ; beak curved ; hinge-margin curved, with stron 
teeth, without area; dorsal y alve with two spirally coiled lamellee, 
the points directed towards the middle of the opposite valve. 
AtTryPA, Dalman, 1828. 
Syn.—Cleiothyris, Phillips, 1841. Spirigerina, d’Orb., 1847. 
Distr.—Fossil, 21 sp. Lower Silurian—Trias; America ( Wel- 
lington Channel! Falkland Islands), Europe, Thibet. .4. reticu- 
laris, Linn. (¢xxxvi, 88-91). 
Shell impunctate ; oval, usually plaited and ornamented with 
squamose lines of growth; dorsal valve gibbose; ventral 
