HUSTEDIA 451 



present it seems best to retain the genus with the genotype as the only 

 representative. 



Horizon. Burlington limestone 



Genus HUSTEDIA Hall 



Description. Shells small, subovate in outline, with nearly equally 

 convex valves, marked by rather coarse, subangular, simple plications, the 

 shell structure punctate. Pedicle valve with a prominent umbo perfor- 

 ated at its apex by a subcircular foramen, cardinal area small, with 

 sharply defined lateral margins, the delthyrium closed by the pseudodel- 

 tidium and in contact with the foramen only at its apex. Internally the 

 inner surface of the pseudodeltidium bears a split tube attached by its 

 closed side to the deltidial plates, with its open side directed towards the 

 interior of the shell. In the brachial valve the hinge-plate projects con- 

 siderably beyond the hinge-line posteriorly, being recurved into the um- 

 bonal cavity of the pedicle valve, the upper face is convex and elevated 

 medially, the posterior margin sinuate and crescentic with the horns of 

 crescent very short, from the lateral margins of the plate arise a pair of 

 strong lobes which bear the erect, slightly recurved crura. At the base 

 of the cardinal process and in the median line, there arises a free, slender, 

 ligulate process which curves upward and backward with a somewhat less 

 curvature than the hinge-plate, and rises to the highest point attained by 

 the latter; the inner surface of this process is deeply grooved and at its 

 base it is supported by a median septum which extends for one-third the 

 length of the valve. The spiral cones of the brachidium and the jugum 

 are similar to those of Eumetria, but the posterior margins of the coils and 

 the jugum are fimbriated, and the extremity of the stem-like process from 

 the jugum is apparently simple. 



Remarks. The genotype of Hustedia is the Pennsylvanian shell H. mor- 

 moni, and the description of the internal characters of that shell, given 

 above, has been taken from] the work of Hall and Clarke. Externally these 

 shells differ from Eumetria only in their smaller size and coarser plications. 

 The internal characters of H. circularis of this report have never been 

 determined, and the species is placed in the genus Hustedia only because 

 of its external resemblance to the genotype. 



HUSTEDIA CIRCULARIS (Miller) 

 Plate LXXVI, Figs. 47-52 



1892. Retzia circularis Miller, Adv. Sheets 18th Rep. Geol. Surv. Ind., 



p. 62, pi. 9, figs. 32-34. 

 1894. Retzia circularis Miller, 18th Rep. Geol. Surv. Ind., p. 316, pi. 9, 



figs. 32-34. 

 1909. Retzia circularis t Weller, Bull. Geol. Soc. Am., vol. 20, p. 317, 



pi. 12, fig. 23. 



