GENERA OF THE SUBORDER ORTHOIDEA 



55 



phores is a great advance over that of other Cambrian 

 forms. 



Cumings'" has suggested the origin of Platystrofhia 

 from Orthis leniicularis {=Orusm). There appear 

 to be no anatomical difficulties in the way of this evolu- 

 tion, yet it is impossible to reconcile with this idea the 

 geological position of these genera in time. Orusui is 

 an Upper Cambrian and possibly a Canadian genus; 

 PLitystroph'w, on the other hand, is known first from 

 the Trenton in this country. There is thus a vast time 

 gap between the disappearance of Orusut and the ap- 

 pearance of Platystroph'ui, a time involving at least all 

 of the Chazyan. The ancestor of Platystrofhia should 

 be looked for in the Chazyan rather than the Cambrian 

 or Canadian. It appears to us, however, that Orusia 

 is ancestral to Fittkelnhurgia and perhaps thus directly 

 in the plectorthid line which, as we interpret the facts, 

 gave rise to Platystrofhia. 



Genus FINKELNBURGIA Walcott 1905, 

 emended 



PI. 13, figs. 6, 7, 9-19; t. fig. 16 



Walcott, Proc. U. S. Nat. Mus., vol. 28, 1905, p. 277; 

 Camb. Brach., 1912, p. 793, pi. 93 (we would select 

 figs. 2 and 2e as representing the genotype). 



Genoholotype. — Finkelnburgia finkeltiburgiWal- 

 cott 1905. 



Description. Exterior. — Subsemicircular to sub- 

 elliptical; hinge-line straight; cardinal extremities 

 acute or obtuse; lateral profile subequally to unequally 

 biconvex. Anterior commissure rectimarginate. Ven- 

 tral palintrope longer than the dorsal one, apsacline; 

 delthyrium and notothyrium open, so far as revealed 

 by the specimens studied, but Walcott erroneously 

 states that the delthyrium is partially covered by a del- 

 tidium. Surface finely multicostellate. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong; dental plates receding; a thickening under 

 the muscles forms a pseudospondylium which is pro- 

 duced into a thick median ridge at the front. Diduc- 

 tor scars divergent, expanding in front; adductor 

 track central, expanded in front, commonly elevated 

 above the diductor scars. Pallial markings two diver- 

 gent trunks sent off from the anterior ends of the 

 diductors, as in Billingsella and Orusia. 



Dorsal interior. — Notothyrial cavity deep ; brachio- 

 phores blunt, fused with supporting plates which con- 

 verge and unite with the floor of the valve beneath a 

 simple cardinal process where such is present. Sockets 

 defined by flattened plates. Adductor muscles borne 

 on callosities; posterior adductor scars the larger; 

 pallial trunks subradial from the adductor field. 



Geologic range. — Upper Cambrian to Canadian 

 of North America. 



Species 



Finkehiburgia finkehihurgi (Walcott) 1905 



F. osceota (Walcott) 1905 



F. Osceola corrugata (Walcott) 190 5 



Eoorthis newtonensis (Wcller) 1903 (in part) 



Dalrnanella wemflei Clcland 1900 



Syntrofhia ormanda (Billings) 1865 



Distinguishing characters. — The distinguish- 

 ing characteristics of Finkelnburgia are its biconvex 

 lateral profile, finely costellate exterior, and pseudo- 

 spondylium and cardinalia like those of Plectorthis. 



Discussion. — In the ventral valve the teeth are 

 stout and are located at the angle formed by the hinge 

 and delthyrial margins. The progressive growth of 



'"Amer. Jour. Sci. (4), vol. 15, 1903, p. 6. 



Fig. 16. — a, c, Finkelnburgia osceola (Walcott). a, ven- 

 tral internal mold, showing musculature and pallial marks, 

 b, dorsal internal mold, b, d, F. pikelnburgi (Walcott). 

 b, ventral, and d, dorsal internal mold. After Walcott 

 1912, pi. 93. iV/*, notothyrial platform; Ps, p.illial sinus; 

 Of, ovarian markings; Di, diductor scars; .4</, adductor 

 scar; Cf, cardinal process. 



the teeth forms a thickening along the margin of the 

 palintrope. The dental plates are attached to the ven- 

 tral surface along the outer side of this ridge, thus 

 making an overhanging shelf in the delthyrial cavity .as 

 in Sken'uiioides. The material forming the pseudo- 

 spondylium is produced forward in front of the adduc- 

 tor tracks as a low thick elevation to about the middle 

 of the v.alve. The adductor track is central, expanding 

 slightly in its forward growth. The diductor tracks 

 are divergent, linear. From the anterior end of each 

 diductor track a strong pallial trunk extends antero- 

 latcrally. In internal impressions these appear as ele- 

 vated, divergent ridges. The ensemble of the ventral 

 internal structure is strongly suggestive of some of the 

 Syntrophiid.-c, but differs chiefly in having a pseudo- 

 spondylium and not a true spondylium. 



