236 



MISSISSIPPIAN BRACHIOPODA 



Description. Shell usually below medium size, broader than long, the 

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

 margins rounded and the anterior margin nearly straight. The dimen- 

 sions of a rather small but nearly complete specimen are: length 14.6 mm., 

 width 17 mm., thickness 12 mm., width of sinus in front 9.5 mm. 



Pedicle valve depressed convex in the posterior half and flattened or 

 gently convex to the antero-lateral margins, along the median line the 

 surface is gently convex to near the anterior margin of the shell where 

 it is strongly curved towards the opposite valve, along the postero-lateral 

 margins towards the beak, the edge of the valve is inflected towards the 

 opposite valve; mesial sinus originating near the middle of the valve as 

 a shallow, flattened depression, it is but little depressed to near the an- 

 terior margin of the valve where it is rather abruptly and strongly curved 







FIG. 22. A series of eight cross-sections of the rostral portion of the shell of 

 Rhynchopora f cooperensis (X 2y 2 ), showing the undivided hinge-plate, and 

 the consequently covered crural cavity. 



towards the opposite valve and produced in a broad lingual extension 

 whose surface lies in nearly a right angle to the plane of the valve, and 

 which is deeply serrate at the distal line of junction with the opposite 

 valve ; beak rather small, pointed, slightly incurved, rather conspicuously 

 protuberant beyond that of the opposite valve, pierced by a small, sub- 

 circular foramen ; delthyrium broadly triangular and communicating with 

 the foramen at its apex, the deltidial plates not observed ; plications sub- 

 angular or rounded, simple and originating at the beak, from four to six 

 occupy the sinus with about six others upon each lateral slope and some- 

 times one or two very faint ones towards the postero-lateral margins, the 

 plications of the sinus are sometimes medially grooved towards the front. 

 Internally the hinge-teeth are supported by dental lamellae which extend 

 anteriorly from the beak for about one-fourth or more of the length of the 

 valve. 



