44 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Table 1 



Porambonitidae 



Dinorthidx 



Plectorthidas 



Lycophoriidse 



Orthidae 



Skenidiidx 



'Eoorthidae 



Protorthidae 



Discussion. — Walcott has always regarded Nisusia 

 as the most primitive known representative of the Pro- 

 tremata, it being also the oldest member of this order, 

 and to this we agree, with the statement that it appears 

 to be the progenitor of the Orthacea. Nisusia appears 

 late in the Lower Cambrian and vanishes with the 

 Middle Cambrian. Jamesella is a European stock of 

 another origin and of the later Cambrian. Possibly 

 out of Nisusia came Protorthis of the Middle and 

 Upper Cambrian, while the older Loperia is of another 

 genetic line. As yet we do not know the origin of 

 Swantonia of the Lower Cambrian, unless it be out of 

 Nisusia, and this doubt is due to lack of knowledge of 

 the dorsal interior. Since Swantonia has no spondylium 

 it may not be an orthid at all, and hence it is not at 

 all certain that this genus gave rise to the Syntrophiacea. 

 Walcott says that Swantonia may have been the pro- 

 genitor of both the syntrophiids and Protorthis^ but in 

 this we can not concur. 



= Camb. Brach., 1912, p. 320. 



Genus NISUSIA Walcott 1905, emended 



PI. l,figs. 16, 20 



Walcott, Proc. U. S. Nat. Mus., vol. 28, 1905, p. 247; 

 Camb. Brach., 1912, p. 725, pi. 100, figs. 1-lj, 

 2-2c, t. fig. 6. 



Genoholotype : Orthisina festinata Billings 1861, 

 Pal. Foss., vol. 1, p. 10, figs. 11-12. 



Description. Exterior. — Subquadrate to semi- 

 oval, unequally biconvex, with the dorsal valve the 

 more convex. Hinge-line straight, usually as great as, 

 or greater than, the greatest width of the shell; car- 

 dinal extremities usually acute, anterior commissure 

 faintly uniplicate; ventral sulcus shallow; ventral 

 interarea broad, strongly apsacline to procline; del- 

 tidium well developed, strongly convex, but only par- 

 tially covering the delthyrium, perforated at the apex. 

 Dorsal interarea anacline, notothyrium more or less 

 well developed. Chilidium present. Surface costel- 

 late, crests of costellas provided with prominent spines 



