248 MISSISSIPPIAN BRACHIOPODA 



eral slopes gently convex and dropping abruptly to the cardinal extremi- 

 ties, anteriorly the valve gradually becomes more broadly rounded trans- 

 versely with less steeply sloping sides, until at the front it is nearly regu- 

 larly rounded from side to side ; mesial sinus wanting ; the beak promi- 

 nent, acuminate, conspicuously produced posteriorly and not incurved; 

 the delthyrium rather large, closed with a pseudodeltidium, the foramen 

 of moderate size, encroaching upon the umbonal region, in contact with 

 the delthyrium only at its apex. 



Brachial valve much shallower than the pedicle, conspicuously sinuate, 

 the sinus originating at the beak and rapidly broadening anteriorly until 

 it occupies nearly the entire width of the valve in front, the most convex 

 portions of the valve are the regions between the median line and the 

 postero-lateral margins, from which regions the surface curves rather ab- 

 ruptly to the cardinal extremities and less abruptly to the lateral margins 

 anteriorly ; the beak acutely pointed and incurved beneath the margin of 

 the pseudodeltidium of the opposite valve. 



Surface of both valves smooth, ordinarily not even exhibiting any lines 

 of growth; the shell structure finely punctate. 



Remarks. This species has been incorrectly identified by Hall and 

 Clarke 1 and their reference of it to the genus Dielasma is erroneous, although 

 the specimen they have illustrated is a Dielasma, and possibly D. formosa. 

 The true shell to which the name Terbratula roivleyi was originally applied 

 is more nearly allied to Centronella in external contour than to Dielasma. 

 The species is conspicuously centronelliform in its external configuration, 

 although it is somewhat thinner with much less incurved pedicle beak 

 than C. glans-fagea the genotype. 



Horizon. Burlington white chert. 



Genus CRANJENA Hall and Clarke 



Description. Shell terebratuliform. Pedicle valve with or without a 

 median sinus, and with well-developed, dental lamella? of moderate length, 

 the foramen large, oblique, and encroaching upon the umbonal portion 

 of the valve, the beak incurved. Brachial valve without median fold, 

 even in those species with a well-defined sinus in the opposite valve, but 

 sometimes with a slight mesial depression near the front margin; inter- 

 nally the well-developed socket plates are connected transversely by a 

 concave hinge-plate which is perforated at the apex of the valve poste- 

 riorly, upon the inner or concave surface of the hinge-plate a pair of 

 ridges originate at or near the anterior margin of the perforation and con- 

 tinue anteriorly across the plate from the front of which they are pro- 

 duced into the crura, these crural ridges upon the hinge-plate divide that 



iPal. N. Y., vol. 8, pt. 2, pi. 81, figs. 27-28. (1894.) 



