604 Foety-fifib Retort on tbe State Mvsevm. 



cavity and extend forward for one-third or one-half the length of 

 the shell. The brachial valve has a full, gibbous umbo,- and a 

 rudimentary or linear cardinal area. On the interior are two 

 divergent lamellae or crural plates, which are extended forward 

 for more than one-third the length of the valve, supporting a 

 high, erect cardinal process, which sometimes reaches almost to 

 the apex of the umbonal cavity in the opposite valve. This pro- 

 cess is a somewhat curved, thin, crescentic plate, bearing on its 

 summit two slender median apophyses, below which are two 

 stouter lateral projections having the form of extended, rounded 

 lobes, which form the bases of the crura. Surface of the valves 

 marked with coarse costae, which, with the interspaces, are covered 

 by fine radiating, often plumose, striae. Shell-substance fibrous, 

 punctate. 



Type, Meehella striatocostata, Cox (sp.). 



Distribution. Upper Carboniferous. 



Streptorhynchus, King. 1850. 

 (Plate 17, Figs. 14-18.) 



Shells somewhat trihedral and subpyramidal, with base convex; 

 hinge-line shorter than the greatest width of the valves. Pedicle 

 valve having the beak acute and often incurved and distorted. 

 Cardinal area high, somewhat incurved ; delthyrium covered by a 

 broad imperforate, depressed-convex deltidium. The interior of 

 the valve with low dental ridges terminating in teeth of moderate 

 size ; these ridges are not septiform and do not continue to the 

 bottom of the umbonal cavit}^ ; there is no median septum ; mus- 

 cular scars strong and flabelliform. 



In the brachial valve the cardinal area is linear, the crural 

 plates are vertical and abruptly elevated on their anterior edges, 

 forming points of attachment for the crura ; these unite to form 

 a subcrescentic plate, from the center of which arises a broad, 

 erect, cardinal process, which is deeply divided into two lobes, 

 (inch of which is faintly grooved at its summit and on the poste- 

 rior face ; muscular area small, divided medial 1\ by a Low ridge. 

 Exterior surface covered by fine radiating strisB. 



Type, St/r&ptorJvynohM8 pelwrgonatus, Schlotheim (sp.). 



Distribution. Carboniferous (?) — Permian. 



156 



