202 MISSISSIPPIAN BRACHIOPODA 



Brachial valve more convex than the pedicle, its greatest convexity 

 near or in front of the middle, the surface curving in nearly a quadrant 

 of a circle from the point of greatest convexity to the beak, with a much 

 gentler curvature to the anterior margin; in the umbonal region the 

 surface is flattened medially and curves abruptly to the postero-lateral 

 margins, from the beak to the antero-lateral margins the curvature of 

 the surface is nearly a semicircle ; the mesial fold originating in front of 

 the middle, of only moderate height at the front margin ; the beak strongly 

 incurved beneath that of the opposite valve; the plications are similar 

 in form and number to those of the opposite valve, there being five on the 

 mesial fold and from seven to ten on each lateral slope, towards the 

 outer margin the plications are sharply angular, alternating with those of 

 the opposite valve, and giving to the line of junction between the two 

 valves a strongly zig-zag direction along the anterior and antero-lateral 

 margins; concentric markings similar to those of the opposite valve. 

 Internally the hinge-plate is divided to the apex of the valve and is 

 entirely unsupported, the cardinal process, median septum, and crural 

 cavity wanting. 



Remarks. This species differs from A. heteropsis, the genotype, in its 

 finer and more numerous plications, a larger number of which occupy the 

 fold and sinus, in the more arched brachial valve from beak to front, in 

 the less incurved beak of the pedicle valve, and usually in the greater 

 proportional width of the shell. 



Locality. Webb City, Mo. ( ?Carterville formation). 



Description. Shell rhynchonelliform, variable in size, the mesial fold 

 and sinus well developed, the sinus sometimes much produced in front, 

 the valves more or less sharply plicated with simple plications, those of 

 the fold and sinus usually the stronger and all becoming obsolete in the 

 posterior portion of the shell, pedicle valve shallow and brachial valve 

 deep. Internally the hinge-teeth of the pedicle valve are supported by 

 well developed dental lamellae. In the brachial valve the hinge-plate is 

 divided, the two portions being unsupported by any lamellae whatsoever, 

 no median septum or cardinal process present. 



FIG. 13. A series of six cross-sections of the rostral portion of the shell of 

 Pugnax pugnus (X 2%), showing the dental lamellae of the pedicle valve and 

 the entire absence of lamellae in the brachial valve. 



