GENERA OF THE SUBORDER ORTHOIDEA 



101 



costellate, fil.-e over the whole surface. Test fibrous, 

 impunctate. 



Ventral interior. — Notothyrial cavity shallow ; teeth 

 small; dental plates small, nearly obsolete; muscle 

 area rhomboidal in outline; wider than long; diductor 

 scars elongate, tear-shaped, expanded slightly in front, 

 not enclosing the adductors; adductor scar oval, 

 slightly elevated, small ; adjustor scars extremely large, 

 occupying the bulk of the muscle-scar. 



Dorsal interior. — Notothyrial cavity very shallow; 

 cardinalia confined; brachiophores short; cardinal 

 process large, keeled on its posterior face, crenulated 

 as in Pla-siomys, protruding above the level of the 

 dorsal interarea; median ridge long. 



Geologic range. — Middle Ordovician (Chazy) 

 of North America. The only known species are 

 P. bella Schuchert and Cooper and P. crassicostella, 

 n. sp. 



Discussion. — This genus is characterized by several 

 features that make it unique. The rafinesquinoid out- 

 line and the fineness of the ornamentation are not char- 

 acteristic of any other group except Multicosiella, and 

 from this genus Planidorsa differs in its lateral profile 

 and in the musculature of the ventral valve. There is 

 no likelihood of confusion of this genus with Valcourea, 

 which diflters from it not only by its extreme change of 

 convexity, but also in certain internal characters. The 

 ventral musculature of Planidorsa is unique for the 

 great expansion of the adjusters, and the small diduc- 

 tors. Another feature of interest is the presence of 

 chilidial plates in the dorsal valve, which are uncom- 

 mon among dinorthids. 



Planidorsa is clearly a dinorthid, as seen in the ven- 

 tral musculature and the structure of the dorsal valve. 

 It is, however, a dinorthid having the external form 

 of Rafinesquina. It is to be expected that the members 

 of the Dinorthidas will show the same modifications of 

 profile as displayed by the Orthidae and Clitambonitidae. 



Planidorsa bella Schuchert and Cooper 1931 

 PI. 16, figs. 24, 25 



Shell rafinesquinoid in outline and profile; ventral 

 valve moderately convex, dorsal valve flat or slightly 

 concave, dorsal valve with a gentle broad sulcus which 

 is lost toward the front. Ventral interarea low, 

 curved, apsacline. Delthyrium open. Hinge-line 

 forming widest part of valve, subalate. In the ventral 

 valve the dental plates are short, the adjustor scars 

 wide, and the diductor impressions elongate. Surface 

 multicostellate, costellae crossed by fine elevated 

 growth-lines, about 5 costelL-c in 5 mm. at the front 

 of the shell. 



Measurements of holotype (Cat. No. 764, 

 Schuchert Collection, Yale University) : Length, 17.5 

 mm.; width, 25 mm. 



Horizon and locality. — Ordovician (Chazy), 

 Waldron, Tennessee. 



Planidorsa crassicostella, n. sp. 



PI. 16, figs. 22, 27 



Differs from P. bella in not being alate and in hav- 

 ing coarser costella:, about 3 in 5 mm. at the front of 

 the shell. 



Holotype, Cat. No. 763, Schuchert Collection, Yale 

 University. 



Horizon and locality. — Ordovician (Chazy), 

 Speer's Ferry, Virginia. 



Family PORAMBONITID^ Davidson, 

 emended 



Aberrant subrostrate Orthacea derived out of Or- 

 thida; and having a biconvex or lenticular profile, 

 narrow hinge-line, an interarea on both valves, open 

 delthyria, and a peculiar ornamentation consisting of 

 flat-topped costella with rows of pits in the interspaces. 



Table 7 



Noetlingia 



Porambonites- 



ORTHIDi^ 



-i Isorhynchus 



Internally there are strong subparallel dental lamella, 

 while the plates of the cardinalia are subparallel or 

 divergent. In gerontic individuals these plates are fre- 

 quently so covered by adventitious shell as to simulate 

 cruralia and spondylia. There is no cardinal process 

 in Porambonites, but there is a simple orthoid one in 

 Noetlingia. Test fibrous, impunctate. 



Geologic range. — Throughout the Ordovician of 

 northwestern Europe. 



The family, although prolific in individuals and 

 species, has but two genera, Porambonites Pander and 

 its side-branch, Noetlingia Hall and Clarke. 



The genetic relations of the family are thought to 

 be as shown in Table 7. 



