GENERA OF THE SUBORDER ORTHOIDEA 



97 



Genus VALCOUREA Raymond 1911 



PI. 10, figs. 16, 19-23, 27-29; t. fig. 2 



Raymond, Ann. Carnegie Mus., vol. 7, 1911, p. 239, pi. 

 35, figs. 15-19, pi. 36, fig. 1, t. fig. 12. 



Genoholotype. — Pltrslomys strophomeno'utes Ray- 

 mond 1905, Amcr. Jour. Sci. (4), vol. 20, p. 370. 



Description. Exterior. — Shells strophomcnoid, 

 hingc-linc wide and straight, cardinal margins rarely 

 suhmucronate or subauriculate, commonly deflected; 

 lateral profile strongly convexo-concave, anterior com- 

 missure broadly uniplicate or faintly sulcate ; dorsal 

 sulcus shallow, in some species obsolete at the front; 

 ventral palintrope broadly triangular, apsacline to cata- 

 cline or slightly procline, delthyrium open or closed by 

 a deltidium; dorsal intcrarea shorter than the ventral 

 one, orthocline or apsacline, notothyrium partially or 

 completely covered by a chilidium; ornamentation 

 multicostellate, fine elevated threads in the interspaces 

 and crossing the ribs. Test fibrous, internally impunc- 

 tate; costells e.xopunctate. 



Ventral interior. — Delthyrial cavity shallow, teeth 

 strong, with accessory sockets (see t. fig. 2) ; crural 

 fossettes oblique, deep; dental supports strong in young 

 shells, obsolete in adults; cavities deep in juvenile 

 individuals. Muscle area subpentagonal, wider and 

 moderately bilobed in front; diductor tracks oblique, 

 elongate, expanded in front; adductor tracks small, 

 thin, elongate; adductor scars elongate, semielliptical; 

 adductor track linear, enclosed by the diductor scars in 

 front; adjuster scars prominent, situated at the base 

 of the dental plates, as in Dinorthis. A small septum 

 may be located at the base of the pedicle callist, or of 

 the deposit under the deltidium. When a deltidium 

 is absent a well marked pedicle callist is present. Pal- 

 lial markings similar to those of Dinorthis, umbo-lateral 

 spaces marked by radiating elevated ridges indicating 

 the position of the ovarian bodies. 



Dorsal interior. — Notothyrial cavity shallow; car- 

 dinalia confined to about the anterior one-fifth of the 

 length of the valve, brachiophores forming the margin 

 of the notothyrium, short, supported by a shell thick- 

 ening beneath; sockets shallow, bounded by the slop- 

 ing face of the brachiophore and an accessory tooth on 

 the hinge margin. Cardinal process large, having a 

 prominent shaft, and a crenulated myophore as in 

 Dinorthis; chilidium present; median ridge short, 

 extending as a rule approximately one-third the length 

 of the shell. 



Geologic range. — Middle Ordovician (Stones 

 River to Black River), chiefly of North America. 



American Species 

 Dinorthis deflecta (Conrad) 1843 

 D.loricula (Hall and Clarke) 1892 

 D. recta (Conrad) 1843 



D. (Valcourea) strofhomenoides (Raymond) 190 5 

 Strofhomena venlrocarinata Butts 1926 

 Valcourea magna, n. sp. 



European Species 

 ? Orthis grandis (Portlock) 



Discussion. — The name Valcourea was proposed 

 by Raymond for "impunctate orthids with reversed 

 valves, strophomenoid habit, well developed deltidium, 

 simple cardinal process, and finely striated [costellate] 

 surface." It was differentiated from Plrrsiomys by its 

 finer ornamentation and the presence of a deltidium 

 throughout life. These orthids form a very remark- 

 able convergence toward the genus Strofhomenn. 

 They may be distinguished therefrom, however, by 

 several important features: Externally they may be 

 distinguished by the resupinate form of Strophomena 

 and the lack of this feature in Valcourea. It is true 

 that the relative convexity of the valves in the latter is 

 reversed, that is, the dorsal valve has the greater con- 

 vexity. However this may be, the umbo of the dorsal 

 valve of Valcourea is always convex and there is a 

 distinct beak. In Strophomena, on the other hand, 

 there is scarcely any dorsal beak and the umbo is 

 decidedly concave. It is the change in convexity from 

 concave at the umbo to convex in the middle and 

 front of the dorsal valve that distinguishes Stropho- 

 mena. The external convergence of Valcourea toward 

 Strophomena is not carried to such a degree that the 

 shell is resupinate, although its convexity is reversed. 

 In the ventral valve the subquadrate plan of the mus- 

 cle field of Valcourea readily separates the two genera. 

 The forked cardinal process of Strophomena is a fea- 

 ture unknown in any orthid. Another difference is to 

 be seen in the deltidia, that of Strophomena being 

 apically perforate and that of Valcourea lacking a 

 foramen. 



Internally, Valcourea and Dinorthis (Pla-siomys) 

 are very similar, differing only in minor details of the 

 pallial markings and ventral musculature. In the 

 latter the diductor scars, although distinctly lobate in 

 front, are not so strongly lobate as those of Valcourea, 

 and the adjustors are much less divergent. The most 

 notable difference, however, occurs in connection with 

 the adductor impressions. Between the diductors of 

 Valcourea is a double ridge which extends to the apex 

 of the reentrant between the diductor lobes. Upon 

 this double ridge are situated the adductor scars which 

 together form an elongate suboval or lanceolate im- 

 pression, widest toward the front. The adductor 

 scars of Valcourea are situated at the front of the mus- 

 cle field; this contrasts with the same impressions in 

 PliFsiomys, which are located in the middle or at the 

 back end of the field. The pallial impressions of 

 Valcourea are much like those of Pla-siomys, but differ 

 in being less distinctly impressed and less broken up 

 into subsidiary rami along the front margins. Fur- 

 thermore, the ovarian radial ridges are larger and 

 more distinctly marked in Valcourea. 



