GENERA OF THE SUBORDER ORTHOIDEA 



137 



Subfamily HETERORTHIN.E Schuchert and 

 Cooper 1931 



Hctcrorthidx with a plano-convex to concavo-con- 

 vex lateral profile and without fold and sulcus. Has 

 only the one known genus, Hrterorth'ts Hall and 

 Clarke. 



Genus HETERORTHIS Hall and Clarke 1892 



PI. 20, figs. 16-21, 25; t. fig. 15 



Hall and Clarke, Pal. N. Y., vol. 8, pt. I, 1892, pp. 202, 

 223, pi. 5b, figs. 20-24. 



Genoholotype. — Orthis clytie Hall 1861, 14th 

 Rcpt. N. Y. State Cab., p. 90. 



Table 12 



HETERORTHINiE HaRKNESSELLIN/E 



Heteroi-this 



Smeathenella 



Harknessella- 



-♦ Reuschella 



Dalmanellio* 



Description. Exterior. — Rafinesquinoid, trans- 

 versely subquadrate, margins convex, cardinal extremi- 

 ties rounded, hinge-line narrower than the greatest 

 shell width; lateral profile depressed plano-convex to 

 concavo-convex; anterior commissure rectimarginate ; 

 ventral interarea the longer, apsaclinc, delthyrium 

 open, umbo expanded, beak incurved; dorsal interarea 

 strongly anacline, notothyrium closed by the cardinal 

 process or a small chilidium ; ornamentation unequally 

 multicostellate, swollen, hollow ribs numerous; shell 

 fibrous, punctate. 



Ventral interior. — Delthyrial cavity shallow; teeth 

 strong, with sockets; dental plates short, flaring, obso- 

 lete in old forms; muscle area large; diductor scars 

 widely divergent, elongate, semiflabellate, not enclos- 

 ing the adductor impressions; adductor scar small, 

 central; pedicle callist large ; adjustor scars elongate, 

 located at the base of the dental plates; around the 

 inside margin of the valve is a thickened rim. 



Dorsal interior. — Cardinalia confined; brachio- 

 phore plates simple, bladelike, divergent, supported by 



adventitious shell deposited at their bases. This secon- 

 dary shell is prolonged for some distanci' into the valve. 

 Sockets wide, deep. Muscle area siibcircuiar ; adduc- 

 tor scars subequal. Cardinal process large, shaft stout, 

 not uncommonly covered by adventitious shell; myo- 

 phorc faintly lobate only at the ventral extremity, 

 elliptical in section in the middle, carinate posteriorly. 

 Dorsal extremity of myophore covered by a small 

 chilidium. Pallial sinuses prominent, two trunks going 

 forward from the anterior end of the median ridge, 

 two obliquely from the antero-lateral margins of the 

 anterior adductor, and a third pair laterally from the 

 space between these muscles. In the dorsal valve there 

 is also a thickening of the shell inside the margin. 



Geologic range. — Middle and Upper Ordovician 

 of United States and Europe. 



American Species 

 Heterorthis clytie {HaW) 1861 



European Species 



Orthis alternata Sowerby 1839 



O. inclyta Barrande 1879 



O. retrorsistria McCoy 1852 



Heterorthis barrandei, nom. nov. (defined below) 



Distinguishing characters. — Heterorthis is dis- 

 tinguished from all other punctate orthids by its trans- 

 versely subquadrate outline; depressed plano-convex 

 or concavo-convex contour; widely divergent diduc- 

 tor scars and very small adductor impressions; con- 

 fined, oblique brachiophores; peculiar cardinal process; 

 small convex chilidium ; and thickened rim around the 

 inside margins of both valves. 



Discussion. — The internal structure of the dorsal 

 valve is probably just as characteristic of Heterorthis 

 as are the elongate diductor scars in the ventral valve. 

 The brachiophore plates are small and bladelike as in 

 Dalmanella, but are enlarged by secondary shell de- 

 posited at their bases and produced forward as a 

 double-pronged hook. The myophore of the cardinal 

 process is elongated ventrally and posteriorly, a modifi- 

 cation that is probably due to the flatness of the valve 

 and parallels Platyorthis in this respect. At the ventral 

 extremity there is a slight trilobation of the myophore, 

 indicating its derivation from a dalmanclloid stock. 

 The trilobed effect is lost chiefly by the posterior pro- 

 longation of the central lobe into a prominent carina. 



Most interesting is the chilidium, which is a rare 

 feature in punctate orthids; it is a small, compressed, 

 cup-shaped plate covering the postero-dorsal surface 

 of the cardinal process. The chilidium takes its origin 

 just inside the brachiophore plates and its outer surface 

 appears to be continuous with the interarea. This 

 covering may have functioned as a protection to the 

 muscle attachments on the median carina of the 

 myophore. 



