NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 281 



served as casts in the sandstone or shale, and none of them have the 

 original shell substance, or a calcareous or siliceous replacement of it. 



A careful examination of a large number of specimens of the dorsal 

 valve fails to reveal a true cardinal process. 



The genus ranges from the Middle Cambrian well up into the Upper 

 Cambrian. The Middle Cambrian species are: Protorthis hlllmgsl^ 

 Protorthis latourensis, Protorthis 7iautes, Protorthis quacoensis^ Pro- 

 torthis spencei. Upper Cambrian: Protorthis? mmnehergeiisis^ Pro- 

 torthis wingi^ Protorthis^ species undetermined. 



The subgenus Lojperia has one species P. (Z. ) dougaldensis from the 

 Middle Cambrian. 



PROTORTHIS BILLINGSI Hartt. 



Ortliis MUingsi Hartt, Acadian Geology, Dawson, 1868, 2(1 ed., p. 644, fig. 223. 

 Orthis Ullingd Walcott, Bull. U. S. Geol. Survey No. 10, 1884, p. 17, pi. i, fig. 



1, la-d. 

 Orthis hilUngai Matthew, Trans. Roy. Soc. Canada, III, 1886, p. 43. 

 (Jrthis ? billingsi Matthew, Trans. Roy. Soc. Canada, VIII, 1891, p. 131. 

 Protorthis billingsi Hall and Clakke, Pal. N. Y., VIII, Pt. 1, pp. 219, 232, pi. 



VII a, figs. 14-20; Eleventh Ann. Rep. State Geologist, New York, 1892, 



p. 273, pi. VIII, figs. 3-7. 

 Billingsella billingsi Schuchert, Bull. U. S. Geol. Sur., No, 87, 1897, p. 158. 



The generic description follows very closely that of the type species 

 P. hiUingsl in its main features. All of the specimens are compressed 

 in the embedding shale, which renders it difficult to get a true concep- 

 tion of the convexity and forms of the valves. The outlines may be 

 transversely quadrilateral or subsemicircular or subquadrate. Usually 

 the area is inclined backward over the hinge line, but it may be distorted 

 by pressure so as to appear to incline forward. Young shells have a 

 well-defined median sinus on the ventral valve that shows as a flattened 

 space on the larger shells. A slight sinus sometimes appears on the 

 dorsal valve. The surface of the shell varies in the number and size 

 of the radiating ribs; sometimes they are scarcely visible toward the 

 cardinal margin, and in other shells they are clearly defined all over 

 the surface; the increase in number is by bifurcation and interpola- 

 tion ; fine, thread-like concentric stride and strong squamose lines of 

 growth cross the ribs. 



Ventral valve moderately convex, with the umbo and apex slightly 

 curved over the area or erect above the area. Area appears to be 

 slightly concave, divided midway by a triangular delthyrium, which 

 is more or less closed by a concave plate that Hall and Clarke consider 

 to be formed by the uniting of an extension of the dental plates or 

 lamellte; from the casts it appears that the teeth were well developed 

 and supported by dental plates that united at the center opposite the 

 delthyrium, but did not reach the bottom of the valve or a median 

 septum; the free spondylium varied in length from a narrow rim 



