228 MISSISSIPPIAN BRACHIOPODA 



in the sum of all its characters, with the genotype, although the punctate 

 structure is much more obscure. It is possible that future investigations 

 of this group of shells will make it necessary to recognize one or more 

 additional genera. 



RHYNCHOPORA HAMBURGENSIS Weller 

 Plate XXIX, Figs. 19-30 



1910. Rkynckopora hamburyensis Weller. Bull. Geol. Soc. Am., vol. 21, 

 p. 515, fig. 17. 



Description. Shell small with valves very unequally convex in mature 

 specimens, triangularly subovate in outline, the postero-lateral margins 

 meeting at the beak in an obtuse angle, the antero-lateral margins 

 rounded, the anterior margin nearly straight or a little concave. The 

 dimensions of two examples are : length 10.2 mm. and 9 mm., width 

 11.1 mm. and 9.8 mm., thickness 8 mm. and 5.5 mm., width of sinus in 

 front 8 mm. and 6.3 mm. 



Pedicle valve shallow, convex in the umbonal region, the surface curv- 

 ing rather abruptly to the postero-lateral margins, gently convex from 

 the umbo to the antero-lateral margins, and strongly arched in adult 

 shells along the median line from the beak to the front margin; mesial 

 sinus originating near the middle of the valve as a broad, shallow, concave 

 depression which becomes deeper anteriorly, not abruptly depressed below 

 the lateral slopes of the valve, produced anteriorly in adult shells in a 

 broad lingual extension rounded in front, whose surface lies in nearly 

 a right angle to the plane of the valve; beak rather broad, not strongly 

 incurved, pierced by a subcircular foramen, rather conspicuously ex- 

 tended posteriorly beyond the opposite valve; delthyrium broadly triangu- 

 lar, partially filled by the beak of the opposite valve, communicating at 



<? O O O 



9 <!> cp CD O O 



J K 



FIG. 20. A series of eleven cross-sections of the rostral portion of the shell of 

 Rhynchopora hamburgensis (X 2%); A to E, the pedicle valve, showing the 

 dental lamellae; F to K, the brachial valve, showing the undivided hinge-plate, 

 the median septum, and covered crural cavity. 



the apex with the foramen which encroaches upon the beak, deltidial 

 plates not observed ; plications simple, rounded or subangular, originating 

 at the beak, from three to six, more usually four or five, occupy the sinus, 

 and about five each lateral slope. Internally the hinge teeth are sup- 

 ported by a pair of short dental plates which quickly become obsolete in 

 front of the articulation of the hinge. 



