238 MISSISSIPPIAN BRACHIOPODA 



postero-lateral margins a little concave, meeting at the beak in an obtuse 

 angle, the lateral margins rounded, the anterior margin emarginate. The 

 dimensions of a nearly perfect specimen, the holotype, are : length of 

 pedicle valve 18 mm., length of brachial valve 17.8 mm., greatest width 

 23.2 mm., thickness 15.6 mm., width of sinus in front 13.3 mm. 



Pedicle valve depressed convex, the greatest depth being posterior to 

 the middle near the umbonal region, the surface curving gently to the an- 

 tero-lateral and lateral margins, becoming more strongly convex to the 

 postero-lateral and cardinal margins; mesial sinus originating near or a 

 little back of the middle of the valve, broad and shallow at first, becom- 

 ing rapidly deeper towards the front where it is produced in an elongate 

 and broad lingual extension directed at about a right angle to the plane 

 of the valve and with a strongly serrate anterior margin ; beak small, 

 sharply pointed, moderately incurved and coming nearly in contact with 

 the umbo of the opposite valve ; the delthyrium broadly triangular, nearly 

 filled by the beak of the opposite valve. The simple plications are rounded 

 posteriorly, becoming more subangular anteriorly, seven or eight occupy 

 each lateral slope of the valve and six the mesial sinus. Internally the 

 hinge-teeth are supported by strong dental lamellae which diverge a little 

 more widely than the lateral margins of the sinus and extend anteriorly 

 more than one-third the length of the valve. 



Brachial valve strongly convex, the greatest depth anterior to the 

 middle, the surface curving abruptly to the antero-lateral, lateral, and 

 postero-lateral margins, becoming inflected to the cardinal extremities; 

 mesial fold originating near the middle of the valve, flattened on top, be- 

 coming only moderately differentiated from the general curvature of the 

 valve towards the front ; the umbonal region prominent, the beak strongly 

 incurved beneath that of the opposite valve. The plications more angular 

 than those of the opposite valve and alternate with them, those reaching 

 the antero-later.al margins being distinctly grooved along their median 

 line for some distance back from the margin. The internal characters of 

 the valve are not sufficiently known, but a thin median septum is present 

 which reaches nearly to the middle of the valve. 



The finer surface markings are not well preserved upon the specimen 

 here described, but they apparently consist of rather fine and somewhat 

 irregular concentric lines of growth which become a little stronger and 

 more crowded towards the front. No punctate shell structure has been 

 observed. 



Remarks. Judging from its external characters alone this species seems 

 to be congerenic with R. persinuata, and the character of the dental 

 lamellae of the pedicle valve and the median septum of the brachial valve 

 also agree with that species. It is not known, however, that the brachial 

 valve possesses an undivided hinge-plate covering the crural cavity, a 

 feature which is characteristic of R. persinuata and other species which 



