BRAOHIOPODA. T75 
External surface of the valves smooth or with concentric striz. 
Shell-structure fibrous, impunctate. 
Type, Merista levis, Hall. Lower Helderberg group. 
Distribution. Devonian. 
Dioristella, Bittner. 1890. 
Smooth shells having a jugum whose lateral branches return 
upon themselves, somewhat as in MrrisTELya. 
Fie. 310.— Jugum of Dioristella indistincta, Beyrich (sp.). (BITTNER.) 
Type, Dioristella indistincta, Beyrich (sp.). 
Distribution. Triassic. 
Charionella, Billings. 1861. 
(Plate 34. figs, 12-14.) 
Shells meristoid in exterior and probably in the structure of 
the brachidium and jugum. Hinge-plate greatly modified, its 
appearance being that of two ridges, strongly recurved at their 
edges, passing along the margins of the delthyrium and inclos- 
ing the dental sockets; these are supported by thin lamellez 
which converge toward the bottom of the valve; the crura aris- 
ing from the extremities of the lateral ridges are short and 
curved outward; the central portion of the plate must be con- 
sidered as absent, or as very concave and merged in the sub- 
stance of the valve. 
Type, Charionella scitula, Hall (sp.) Corniferous limestone. 
Pentagonia, Cozzens. 1846. 
(Plate 34, figs. 15-20.) 
Synonym; Goniocelia, Hall, 1861. 
Shells with a very broad sinus on the pedicle-valve, which is 
limited by divergent carinz, outside of which the cardinal or 
lateral slopes are very abrupt. On the brachial valve is a 
rounded median fold, which may be divided on its summit by a 
narrow sinus, and in the umbo-lateral region are two short folds 
27 
