MISSISSIPPIAN LOOP-BEARING BRACHIOPODA 445 



by a concave hinge-plate which is perforated at the apex of the 

 valve posteriorly; 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 continue anteriorly across the plate, from the 

 front of which they are produced into the crura. These crural 

 ridges divide the hinge-plate into three equal divisions, or into two 

 equal lateral divisions and a broader central one, and in some 

 species the crural ridges are accompanied by similar ridgelike 

 thickenings upon the opposite side of the hinge-plate. The 

 brachidium short and Dielasma-like, not reaching to the middle 

 of the valve. 



Remarks. — This genus differs fundamentally from Dielasma 

 in the origin of the crura from the thickened crural ridges of the 

 hinge-plate, rather than from crural lamellae resting upon the 

 inner surface of the valve, and in the absence of a special plate in 

 the brachial valve for muscular attachment, the muscles being 

 attached directly to the inner surface of the valve. From Girty- 

 ella the genus differs in the perforation of the hinge-plate, in the 

 absence of a median septum in the brachial valve, and in the 

 origin of the crura from crural ridges of the hinge-plate rather than 

 from the anterior extension of the inner extremities of the socket- 

 plates. 



Genotype. — C. romingeri (Hall). Other species, C. iowensis 

 (Calvin), C. n. sp., residual chert, Springfield, Mo. 



Hamburgia n. gen. 



Description. — Shell terebratuliform. Pedicle valve not sinuate 

 in the genotype and only known species, the foramen large, oblique, 

 encroaching upon the umbonal region; brachial valve without 

 fold or sinus. Internally the pedicle valve is supplied with well- 

 developed dental lamellae; the brachial valve with well-developed 

 socket-plates which retreat from the lateral margins of the valve 

 anteriorly beyond the articulation of the valves; they are con- 

 nected transversely by a deeply concave hinge-plate which, is 

 separated from the inner surface of the valve by an exceedingly 

 low and broad cavity; upon the inner or concave side of the 

 hinge-plate a pair of ridges originate toward the apex and diverge 



