46 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



direction, and the margins of the interarea overlap 

 them, leaving a long groove on their outer side which 

 serves as the dental sockets. A chilidium is present in 

 several species, but there is no cardinal process. The 

 only evidence of the adductor tracks is tvi^o elongate, 

 divergent depressions just anterior to the notothyrial 

 callosity. 



From the above it is evident that it has been neces- 

 sary wholly to revise Walcott's list of species referred 

 to Nisusia, with the result that but four are left unques- 

 tionably by us in this genus. N. comfta (Tate) of 

 South Australia varies widely from Nisusia in the shape 

 of the valves. It is syntrophiid in form and suggests 

 A^. oriens Walcott 1924. Both these species should be 

 referred to another genus. 



Genus JAMESELLA Walcott 1905 



PI. 1, fig. 24 



Walcott, Proc. U. S. Nat. Mus., vol. 28, 1905, p. 252; 

 Camb. Brach., 1912, p. 731, pi. 101, figs. 1-lh. 



Genoholotype. — Orthis ferfasta Pompeckj 1896, 

 Jahrb. geol. Reichs. Wien, vol. 45, pp. 515-516, pi. 

 15, figs. 15-18. 



Description. Exterior. — Transversely subquad- 

 rate; hinge-line straight, cardinal extremities usually 

 obtuse. Lateral profile subequally biconvex. Fold and 

 sulcus not stabilized. Ventral interarea long, curved, 

 strongly apsacline; delthyrium open or covered by a 

 convex deltidium. Dorsal interarea short, moderately 

 anacline; notothyrium open. Surface usually multi- 

 costellate. Microstructure of shell unknown. 



Ventral interior. — Delthyrial cavity deep, dental 

 plates rudimentary ( ? ) ; muscle area wide ; adductor 

 track subtriangular, longer than the diductor scars; 

 diductor impressions wide, attached on the sides of the 

 delthyrial cavity; pallial depressions on the outside 

 margins of the diductors, but their anterior continua- 

 tions are not visible. 



Dorsal interior. — Notothyrial cavity moderately 

 deep, brachiophores billingselloid, dental sockets small; 

 cardinal process linear or rudimentary; adductor scars 

 unknown. 



Geologic range. — Walcott (1912) says that 

 Jamesella occurs in the Lower Cambrian, but the 

 actual deposits having the conglomerate pebbles with 

 fossils are of Middle Cambrian time. The genus is 

 very poorly known and none of the American species 

 referred to it conforms to the structure of the geno- 

 type. As here redefined, Jamesella is restricted to the 

 Middle Cambrian of Bohemia, most of the American 

 Lower Cambrian species placed here by Walcott being 

 better referred doubtfully to Nisusia. 



European Species 



Nisusia {Jamesella) kuthani (Pompeckj) 1896 



TV. (7.) ferfasta (Pompeckj) 1896 



N. (j.) fer-pasta macra (Pompeckj) 1896 



N. (J.) ferfasta subquadrata (Pompeckj) 1896 



Distinguishing characters. — Walcott distin- 

 guished Jamesella from Nisusia chiefly by the absence 

 of external spines in the former. More fundamental, 

 however, are the presence of pronounced muscle-scars 

 showing a rather wide adductor track, and the rudi- 

 ments of pallial marks similar to those usually seen in 

 Billingsella. In the dorsal valve the cardinalia are 

 billingselloid, so far as can be determined from squeezes 

 of the genotype specimens. The interior characters are 

 those rather of Middle Cambrian than of Lower 

 Cambrian time. 



Family PROTORTHID^ Schuchert and 

 Cooper 1931 



Primitive specialized Orthacea with a very short free 

 concave plate, evidently a free spondylium; there are 

 no dental plates nor is there a deltidium or chilidium; 

 the cardinalia are like those of Billingsella, and a car- 

 dinal process is absent. 



The family embraces Protorthis Hall and Clarke 

 and Loperia Walcott, both of the Middle Cambrian of 

 New Brunswick and Nova Scotia. 



Discussion. — Walcott (1912, p. 317) thinks that 

 Protorthis arose in the Lower Cambrian out of the 

 stock that gave rise to Nisusia, and to this we agree. 



Genus PROTORTHIS Hall and Clarke 1892 



PI. 1, figs. 12, 14 



HaU and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 231, 



pi. 7a, figs. 14-21. 

 Walcott, Camb. Brach., 1912, p. 738, pi. 99, figs. 1-lg. 



Genoholotype. — Orthis billingsi Hartt 1868, in 

 Dawson, Acad. Geology, 2d ed., p. 644, fig. 223. 



Description. Exterior. — Shell usually small, thin- 

 shelled, transversely subquadrate or semicircular, hinge- 

 line straight, usually equal to the greatest width of the 

 valves; cardinal extremities angular; lateral profile 

 subequally biconvex. Ventral interarea long; delthy- 

 rium open. Dorsal interarea short; notothyrium open. 

 Surface multicostellate. Microstructure fibrous?, 

 impunctate. 



Ventral interior. — Teeth small, delthyrial cavity de- 

 fined by a free spondylium, probably for pedicle mus- 

 cle attachment. Muscle-scars not visible on spoon or 

 floor of valve. 



Dorsal interior. — Notothyrial platform very shallow, 

 confined and without a cardinal process; chilidium 

 absent; brachiophores like those of Billingsella, short; 

 median ridge absent. 



Geologic range. — Middle Cambrian of New 

 Brunswick. 



