98 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Valcourea appears to be best represented in the lower 

 Middle Ordovician of North America where the genus 

 first appears in the Stones River group and is last seen 

 in the Black River. The range is short but the species 

 are widely spread during this time. In Europe one 

 species, Orthis grandis Portlock, seems to belong in this 

 association. Valcourea is contemporaneous with D'm- 

 orthts s. 5. and PlmsiomySy but dies out long before 

 either of these tribes. 



Valcourea magna, n. sp. 

 PI. 10, figs. 16, 20, 27-29 



Shell large, wider than long, convexo-concave to 

 convexo-plane ; dorsal valve having a faint sulcus 

 which is lost toward the front of the shell. Ventral 

 interarea long, strongly apsacline. Delthyrium open. 

 Surface multicostellate, costellae crossed by fine ele- 

 vated concentric lines. Ventral muscle field bilobed in 

 front, pentagonal in outline. Pallial marks as in 

 Dinorthis. 



This species is the largest of all the Valcoureas and, 

 so far as known, does not have a deltidium. The con- 

 cavity of the dorsal valve is less than is usual in other 

 species. 



Measurements of the holotype, Cat. No. 779, Schu- 

 chert Collection, Yale University: 



Length Thickness 



37 mm. 17 mm. 



Width 

 47 mm. 



Horizon and locality. — Ordovician (Simpson), 

 Criner Hills, Oklahoma. 



Genus MULTICOSTELLA Schuchert and 

 Cooper 1931 



(Lat. multiy many; costella, small rib) 



PI. 8, figs. 19,22,23,27; pi. 15, fig. 12 



Schuchert and Cooper, Amer. Jour. Scl. (5), vol. 22, 1931, 

 p. 244. 



Genoholotype. — Orthis ( ? ) saffordi Hall and 

 Clarke 1892, Pal. N. Y., vol. 8, pt. 1, pp. 217, 340, 

 pi. 5 A, figs. 38-40. 



Description. Exterior. — Shells semielliptical, an- 

 terior margin convex; lateral margins convex or 

 straight; hinge-line wide and straight; cardinal ex- 

 tremities angular or obtuse; lateral profile subequally 

 biconvex; anterior commissure rectimarginate to sul- 

 cate; fold and sulcus absent, faintly or strongly devel- 

 oped; interareas nearly equal in length, the ventral 

 usually slightly longer; ventral interarea apsacline, 

 umbo convex, delthyrium open; dorsal interarea or- 

 thocline or faintly anacline; umbo very gently con- 

 vex, usually sulcate. Ornamentation multicostellate, 

 interspaces with elevated growth-lines; test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity shallow; teeth 

 small; dental plates strong, advancing; small pedicle 

 callist in the apex; crural fossettes shallow, oblique; 

 muscle area nearly one-third the length of the shell, 

 about as long as wide, bilobed in front; diductor scars 

 elongate, expanded in front; adductor tracks elon- 

 gate; adjustor scars short, narrow, slightly expanded 

 in front. Elevated, elongate oblique lines occupy the 

 lateral spaces next the cardinal cavities. 



Dorsal interior. — Notothyrial cavity shallow; sock- 

 ets shallow; brachiophores stout, blunt, supported by 

 the thickening of the notothyrial platform; cardinal 

 process stout, myophore crenulated ; median ridge low, 

 short; adductors not deeply impressed on the shell. 



Geologic range. — Middle Ordovician (Chazy) 

 of North America. 



Species 



Dinorthis flatys (Billings) 1859 



Plusiomys brevis Willard 1928 



P. «/07z^d/<j Willard 1928 



Orthis ? saffordi Hall and Clarke 1892 



Discussion. — Multicostella is most closely allied to 

 Valcourea and Dinorthis but is distinguished from both 

 by its subequally convex valves and subequal interareas. 

 Although a faint resupination of the ventral valve can 

 be detected frequently in Multicostella, this feature is 

 never developed to the extreme degree seen in Val- 

 courea and Dinorthis. None of the observed specimens 

 of the genus shows any evidence of a deltidium, but 

 there is always a well marked pedicle callist in the 

 apex; the absence of the deltidium in itself is not 

 thought to be a generic character but its constant 

 absence when combined with other peculiarities of the 

 shell is of considerable importance. The pedicle mus- 

 cle callist is a small thickened wrinkled area in the 

 apex similar to the one of Valcourea when the del- 

 tidium is absent. Multicostella also differs from Din- 

 orthis in the fineness of its radial ornamentation. 



Internally the variations from Dinorthis and Val- 

 courea are not especially striking. Perhaps the most 

 notable one is the very moderate development of the 

 adjustor muscles, the marked size of which in the other 

 two genera has a marked effect on the dental plates 

 and the shape of the muscle area. When the adjusters 

 are greatly enlarged they crowd the dental plates out- 

 ward toward the hinge-line, restricting the umbonal 

 cavities and preventing any notable forward growth of 

 these plates. Consequently in Valcourea the umbonal 

 cavities are less pronounced than in Multicostella and 

 do not ordinarily stand out as sharply defined plates. 

 Furthermore, the expansion of the adjustor scars 

 widens the muscle area and gives it a pentagonal out- 

 line. The dorsal valves of the two genera are in essen- 

 tial agreement. In Valcourea, however, there is a 

 tendency toward the filling up and obliteration of the 



