GENERA OF THE SUBORDER ORTHOIDEA 



141 



Hebertella of the Plectorthin.r. Schizophorclla is 

 the lenticular type of hebertelloid evolution, just as 

 Piotwderna represents the same phase in the develop- 

 ment of Sch'tzofhor'ut. The convexo-concave profile 

 of Hebertella corresponds to the similar shell form in 

 Schizophoria. 



Genus PIONODEMA Foerste 1912 

 PI. 23, figs. 1-10, 12-14; pi. 29, fig. 1 



Foerste, Bull. Denison Univ., vol. 17, 1912, p. 139. 

 Cooper, Jour. Pal., vol. 4, 19.30, pp. 369-382. 

 Horn. Bathyc<xlui Foerste 1909. 



Genoholotype. — Orthis subtrquata Conrad 1843, 

 Proc. .Acad. Nat. Sci. Phila., vol. 1, p. 333; Cooper, 

 op. cit., pi. 35, fig. 4, pi. 36, figs. 1, 3, pi. 37, fig. 6. 



TabU 14 



Orthotichia ■ 



-> Aulacophoria 



->Enteletin« 



Schizophoria *. 



-^ ISORTHIN^E 



' Pionodema 



Description. Exterior. — Semi-oval or subglobose, 

 margins convex, cardin.-il extremities obtuse, hinge-line 

 slightly narrower than the greatest width of the shell, 

 lateral profile lenticular or globose, the anterior part 

 of the ventral valve becoming concave at the front; 

 anterior commissure unisulcate or uniplicate; ventral 

 interarea longer than the dorsal, gently curved, apsa- 

 cline, beak slightly incurved, umbo prominent; dorsal 

 interarea curved, orthocline, umbo convex, sulcate; 

 delthyria open or partially closed by an apical plate; 

 multicostellate, with hollow costellx; shell fibrous, 

 punctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 small, triangular; crural fossettes oblique; dental 

 plates sharply defined, widely divergent, continued for- 

 ward as a slight ridge of adventitious shell for a short 

 distance along the lateral margin of the muscle area; 



muscle area not confined to the delthyrial cavity, 

 longer than wide; diductor scars divergent, subsemi- 

 flabellate ; adductor impressions consisting of two elon- 

 gate semielliptical scars on a small ridge between the 

 diductors; adjustor scars elongate, situated at the base 

 of the dental plates. Apex closed by a small apical 

 plate, the front of which is bevelled slightly below the 

 level of the interarea. This plate clearly served for the 

 att.ichment of the pedicle. Ovarian impressions occupy 

 the umbo-lateral spaces. 



Dorsal interior. — Cardinalia confined ; brachiophore 

 supporting plates not separable, vertical or nearly so, 

 divergent at their bases; brachiophores sharp, diver- 

 gent, shaped like a boar's tusk; sockets small, excavated 

 beneath the palintrope, defined by a small concave ful- 

 cral plate which also serves to bind the brachial appara- 

 tus to the wall of the valve; cardinal process very 

 small, expanded and bilobed posteriorly, its shaft being 

 extended forward to merge with the low median ridge 

 that extends forward nearly to the middle of the shell. 

 Muscle area very faintly impressed, anterior adductor 

 scars the larger. 



Geologic range. — Early Middle Ordovician 

 (Chazy) to Upper Ordovician (Maysville). 



American Species 



Dalmanella bellula (Meek) 1873 



Z). «>c«/<jriV (Winchell) 1880 



D.conradi (Winchell) 1880 



D. iubtxquata (Conrad) 1843 



Pionodema minnesolensis Cooper (Jour. Pal., vol. 4, 1930, 



pp. 374, 379, pi. 36, fig. 14) 

 P. uniflicata Cooper (Ibid., pp. 374, 379, pi. 35, figs. 1,2) 



European Species 

 Orthis redux Barrande 1847 



Distinguishing characters. — The diagnostic 

 characters of Pionodema are the globose lateral profile, 

 long ventral interarea, ventral musculature, and espe- 

 cially the structure of the cardinalia, which is most like 

 that in Schizophoria, Orthotichia, and Enteletes. 



Discussion. — Foerste separated this group in 1909 

 under the name Bathyca-ita but later changed it to 

 Pionodema when he learned that the former name 

 was preoccupied. There was no description of the 

 genus, the author merely stating that Dalmanella 

 suhcequata typified the group and that it ranged from 

 the Stones River to the Devonian. It was unfortunate 

 that the genus was so loosely defined, as it now makes 

 great difficulty in the correct interpretation of the 

 genotype. It has been shown in the discussion of 

 Doleroides (p. 63) how shells of external character 

 nearly identical to that of P. subtequata actually differ 

 very markedly internally and can not be referred even 

 to the same family. It becomes necessary, therefore, 

 to give an analysis of the type of Orthis subcsquata 

 and from this to build a proper definition of the genus. 



