60 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Description. Exterior. — Shells large or small, 

 subelliptical or subquadrate ; hinge-line straight, usually 

 narrower than the greatest width; cardinal extremi- 

 ties rounded or angular, occasionally deflected; lateral 

 profile convexo-concave or unequally biconvex; an- 

 terior commissure uniplicate, uncommonly sulcate; 

 anterior margins rarely emarginate. Ventral palin- 

 trope generally strongly apsacline, slightly curved; 

 dorsal palintrope shorter than the ventral, orthocline or 

 apsacline, curved; ventral beak slightly incurved; 

 dorsal beak arched over the ventral interarea, umbo 

 gently convex or inflated; exterior paucicostate to 

 multicostate with fine concentric ornamentation ; swol- 

 len tubulose costffi. Test fibrous, internally impunc- 

 tate, scattered exopunctas on the surface. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong, with lateral sockets, crural fossettes oblique, 

 moderately deep ; dental plates strong ; umbonal cavi- 

 ties deep, muscle field obcordate, strongly impressed, 

 bounded by an elevated ridge extending from the 

 anterior ends of the dental plates; diductor scars sub- 

 crescentic, not enclosing the adductor track in front; 

 adductor scars elongate suboval, borne on a double 

 median ridge which has a shallow groove in the center ; 

 adjustor scars obscure at the base and on the sides of 

 the dental plates; pedicle callist visible in some shells, 

 small, slightly elevated above the floor; ovarian mark- 

 ings consisting of interrupted wavy elevated lines, most 

 prominent on each side of the muscle field. 



Dorsal interior. — Notothyrial cavity deep, brach- 

 iophores margining notothyrium, divergent, bluntly 

 pointed, supported by convergent plates which unite 

 with the roof of the valve; socket deep, marked by a 

 small fulcral plate; cardinal process a thick ridge, 

 median thickening extending to the center of the 

 valve; muscle marks obscure, posterior pair larger than 

 the anterior ones; pallial markings a few oblique, 

 radial lines from the crural cavities. 



Geologic range. — Middle and Upper Ordovician 

 (Chazy? to Richmond) of North America. 



Species 



Heberlella alveata Foerste 1909 



H. alveata richTnondensis Foerste 1909 



H. jrankjortemii Foerste 1909 



H . latasulcata Foerste 1914 



//. OTarw (Billings) 1862 



H. maria ■parkensis Foerste 1909 



H . occidentalis (niM) 18+7 



H . occidentalis siiiuata (Hall) 1847 



H.subjugata (Hall) 1847 



H . clermontensis Bradley 1 92 1 



(looks like Dinorthis froavita) 

 ? H. lonensis (Walcott) 1884 



Distinguishing characters. — Hehertella is char- 

 acterized externally especially by the convexo-concave 

 or unequally biconvex profile of the valves and the 



multicostellate ornamentation. Internally the plan of 

 the ventral muscle field with the double adductor ridge 

 is unique. In the dorsal valve the cardinalia are as in 

 Plectorthis. 



Discussion. — The ventral muscle field is character- 

 ized by wide diductor scars having thickened inner 

 margins which form a double central ridge. Upon 

 this ridge are located the adductor muscles, forming 

 together an elongate oval scar. The adjustor marks 

 are located on the sides of the dental lamellae, where 

 they join the valve and usually escape notice. The 

 other margins of the muscle area are commonly ele- 

 vated strongly, forming a deep basin. In old shells 

 a callus is deposited at the back end of the shell, 

 obliterating all the muscle impressions in its growth 

 forward (see pi. 11, fig. 24). In the apex is a rather 

 small pedicle callist, commonly elevated slightly above 

 the floor of the valve. In some instances it is so small 

 as to be of doubtful value in the attachment of the 

 pedicle. Not uncommonly a double-ridged median 

 elevation develops from the anterior end of the callist 

 and grows forward, in some instances for half the 

 length of the muscle field. It is a common phenom- 

 enon of H. occidentalis sinuata to see one or two slight 

 median ridges extending for a short distance in front 

 of the median double muscle track. It is a striking 

 fact that Hehertella shows no conspicuous pallial mark- 

 ings except an irregular pitting of the surface, and, in 

 the vicinity of the umbonal cavities, interrupted, ele- 

 vated lines or ridges. 



The dorsal interior is essentially the same as that of 

 Plectorthis. In old shells the cardinal process may 

 become bulbous in front but the myophore remains a 

 thin, bladelike ridge crenulated on its sides. The pos- 

 terior adductor impressions are the larger and in old 

 age are longitudinally ribbed. As alluded to previ- 

 ously, Hehertella and Plectorthis are very closely re- 

 lated. Young forms of Hehertella can scarcely be dis- 

 tinguished from some mature species of Plectorthis, 

 either externally or internally (see pi. 11). This 

 genus may or may not be the ancestor of Hehertella; 

 it is impossible to determine this until the Chazy 

 "Plectorthis" are more seriously investigated. How- 

 ever, the two lines have a common origin and certainly 

 belong in the same family, but each genus may have 

 arisen independently out of Finkelnhurg'ut. 



As conceived by Hall and Clarke, the genus Hehert- 

 ella comprised a heterogeneous assemblage of shells 

 including such genera as Glyftorthis, Eridorthis, and 

 Austinella. It has already been shown that Glyftor- 

 this and Eridorthis are wholly unrelated to Hehertella 

 and belong in a different family. 



Hehertella appears to be confined to the Ordovician of 

 North America. Reed has referred certain Girvan shells 

 to this genus but none of them agree with Hehertella. 

 The species H. halclatchiensis and H. crisfa appear 

 to belong to Glyftorthis and consequently are of a 



