48 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



cardinalia are more strongly developed and there is a 

 low thick cardinal process; nothing of muscle marks 

 was seen. 



Discussion. — Walcott states that the shell of this 

 genus is punctate, and the evidence is based on the pres- 

 ence of papillae on the internal and external molds. If 

 these indicate endopunctas, they should be confined to 

 the internal molds; besides, they are coarser than is 

 common for punctas in punctate shells. We can not 

 explain these supposed pores either as fortuitous or as 

 anatomical features. We incline toward the former 



Family BILLINGSELLID^ Schuchert 1893 



Primitive costellate Orthacea, usually with a deltid- 

 ium and chilidium; there is no spondylium, since the 

 dental plates remain discrete, but there may be a pseu- 

 dospondylium. The ventral muscles are clearly differ- 

 entiated into broad adductor and well marked diductor 

 tracks. Ventral pallial sinuses widely divergent. In 

 the dorsal valve the brachiophores are short, flat, 

 oblique plates, placed under the palintrope. Cardinal 

 process present or absent ; when present, a simple ver- 

 tical plate. As now constituted, the family has but 

 the one genus, Billingsella. 



Discussion. — Hall and Clarke* say that Billingsella 

 "may have served as a point of departure for the 

 Orthidas and Strophomenidae." Walcott'' is correct in 

 saying that the general resemblance of the Cambrian 

 eoorthids to certain Ordovician Protremata is striking 

 and the lines of descent suggestive. This is seen not 

 only in the family Billingsellidae, but as well in its 

 descendants the Finkelnburgiida:, Plectorthidas, Or- 

 thidas, and the Clitambonacea and Syntrophiacea. 

 The Strophomenacea are structurally foreshadowed 

 here, but these apparently did not originate directly out 

 of the Billingsellidas but out of one of the later families, 

 probably the Orthidas, during the Lower Ordovician, 

 since the earliest known forms occur in the Middle 

 Ordovician (Chazy). In this work we have not taken 

 up a revision of the Strophomenacea for want of time, 

 and must leave it to others. 



Billingsella, as here defined, could, theoretically, 

 have given rise to the Clitambonacea, a group now 

 known to be closely related to Orthis. This super- 

 family is provided with pallial markings exactly like 

 those of Orthis (see pi. 8, fig. 10). Furthermore, the 

 Clitambonitidas and Deltatretidas retain the primi- 

 tive deltidium and chilidium so well developed in 

 Billingsella. 



«Pal. N. Y., vol. 8, pt. 2, p. 355. 

 ' Camb. Brach., p. 300. 



Genus BILLINGSELLA Hall and Clarke 1892 



PI. 1, figs. 6, 10, 13, 19, 21, 25, 27; pi. 29, 

 figs. 12, 13 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 230, 



pi. 7, figs. 16-19, pi. 7a, figs. 7-9. 

 Walcott, Camb. Brach., 1912, p. 7+9, pi. 85, fig. 1, t. 



figs. 6, 66. 



Genoholotype. — Orthis fefina Hall 1863, N. Y. 

 State Cab., 16th Rept., p. 134, pi. 6, figs. 23-27. 



Description. Exterior. — Subquadrate or semi- 

 circular in outline, ventral valve subquadrate, dorsal 

 valve usually transverse; hinge-line straight; cardinal 

 extremities usually rectangular or obtusely angular, 

 occasionally acute; lateral profile unequally biconvex; 

 anterior commissure faintly sulcate ; ventral palintrope 

 very long, orthocline or moderately apsacline; delthy- 

 rium wide, more or less covered by a convex deltidium 

 which in some species is perforated at the apex by a 

 small aperture. Dorsal palintrope shorter than the 

 ventral, strongly anacline, notothyrium wide, partially 

 covered by a convex chilidium. Surface multicostellate. 

 Microstructure fibrous, impunctate. 



Ventral interior. — Teeth prominent and large, with 

 strong sockets on their outer margins; dental plates 

 strong, oblique to the vertical and widely divergent; 

 musculature prominent and scarcely ever making a dis- 

 tinct pseudospondylium ; diductor tracks widely diver- 

 gent; adductor track forming a central triangular scar 

 that is often more or less prominently elevated in front. 

 Pallial marks prominent, consisting of two pairs of 

 divergent, narrow sinuses, originating at the anterior 

 margin of the diductors and extending forward about 

 three-fourths the length of the valve, where they fork. 

 In the center of the valve they define an elongate 

 pentagonal area and laterally they bound a subreniform 

 space. At the anterior margin innumerable secondary 

 sinuses extend radially toward the margin. 



Dorsal interior. — Brachiophores widely divergent, 

 short, and with imperfectly defined sockets appearing 

 as shallow cups outside the crural bases. Brachiophore 

 bases usually somewhat obscured by adventitious shell 

 grown about them, forming a prominent thickening in 

 the notothyrial cavity and extending forward a short 

 distance as a median axial thickening. Cardinal process 

 a simple ridge between the diductor scars; adductors 

 elongate ; elliptical scars diverging from the prominent 

 pits anterior to the notothyrial platform; a radial pallial 

 mark extending with slight convexity from the anterior 

 end of each adductor impression. 



Geologic range. — Mainly Middle Cambrian to 

 uppermost Cambrian. There is also an undescribed 

 species in the lower Ozarkian of North America. 



American Species 

 Billingsella ? affalachia Walcott 1905 

 B. color ado ensis (Shumard) 1860 

 B. holtedahli Walcott 192+ 



