562 CAMBEIAN BRACHIOPODA. 



area. ■ A comparison of the interiors of Oholus apoUinis Eichwald (PI. VII, figs. 1-17) with those 

 of Oholus (Fordinia) hellulus (Walcott) (PI. LI, figs. 3a-f) and ETkania desiderata (BiUings) (PI. 

 LI, figs. 1, la-d) illustrates how little change is necessary to convert the exterior cardinal areas 

 of Oholus into the inclosed internal areas of Elkania. 



As the beaks of Elkania are marginal, the pedicle passed out tlirough an opening made by 

 the gaping of the valves. 



The tliree known species of the genus are the type species Elkania desiderata (Billings), E. 

 amhigua (Walcott), in which the shell is very thick, and E. ida (Billings). 



Authors have referred frequently to the resemblance between species of Oholella and Elkania 

 desiderata. This does not appear to exist except in a superficial manner. Oholella is a stage 

 in the evolution toward the Siphonotretidffi, whereas Elkania has no known descendants. 



The generic name was given in honor of Dr. Elkanah BiUings, of the Geological Survey of 

 Canada. 



Elkania ambigua (Walcott). 



Plate LI, figures 2, 2a-c. 



Oholella f ambigua Walcott, 1884, Mon. U. S. Geol. Survey, vol. 8, pp. 67-68, PI. I, figs. 2a-c. (Described and discussed 

 as a new species. The specimens represented by figs. 2a-c are redrawn in this monograph, PL LI, figs. 2a, 2, and 

 2b, respectively.) 

 Billingsia f ambigua (Walcott), Ford, 1886, Am. Jour. Sei., 3d ser., vol. 31, p. 467. (Generic relations mentioned.) 

 Elhania ambigua (Walcott), Foed, 1886, idem, vol. 32, p. 325. (Merely changes generic reference.) 

 Elkania ambigua (Walcott), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 78. 

 (Generic relations discussed.) 



General form ovate; biconvex; beaks marginal. Surface marked by fine concentric strise 

 of growth. Substance of shell corneous as far as can be determined, although some of the 

 layers appear to be calcareous. The shell is made up of many thin layers or lamellae. 



Ventral valve elongate oval, moderately convex. Dorsal valve transversely broad ovate; 

 the interior shows a slender median septum at the bottom of a shallow, concave, well-defined 

 visceral area; within the latter area the central muscle scars occur at the outer margins about 

 one-half the distance from the posterior margin to the anterior margin of the visceral area; 

 the anterior lateral scars are close together on each side of the median septum at the anterior 

 margin of the area; the thickening of the shell varies greatly in different shells; in some it 

 extends far to the front (PL LI, fig. 2c) and in others it is scarcely noticeable (fig. 2b). 



No interiors of the ventral valve have been found and the cardinal area of the dorsal valve 

 is unknown. The largest dorsal valve has a length of 9 mm.; width, 10 mm. 



This species in size and form is related to E. desiderata (Billings) ; it differs in the character 

 of the interior of the dorsal valve, and in having a thicker and stronger shell in the adult stage. 



When the species was first described its relations were uncertain, hence the specific name. 



Formation and locality. — Lower Ordovician : (63) Base of the Pogonip limestone, northeast of Adams Hill; and 

 (202) Pogonip limestone, on the summit of the ridge directly southeast of the Jackson mine, northwest of Shadow 

 Canyon; both in the Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



Elkania desiderata (Billings). 



Plate LI, figures 1, la-d. 



Obolella desiderata Billings, 1862, Geol. Survey Canada, Paleozoic Fossils, vol. 1, pp. 69-70, figs. 62a-b. (Described 

 and discussed as a new species. The specimens represented by figs. 62a and 62b are redrawn by Hall and 

 Clarke, 1892c, PL III, figs. 16 and 17, respectively.) 

 Obolella desiderata Billings, Davidson, 1868, Geol. Mag., vol. 5, figs. 1 and 2, p. 809. (Mentioned.) 

 Obolella f desiderata Billings, Walcott, 1886, Bull. U. S. Geol. Survey No. 30, p. 111. (Generic relations discussed.) 

 Billingsia desiderata (Bilhngs), Ford, 1886, Am. Jour. Sci., 3d ser., vol. 31, pp. 466 and 467, fig. 1, p. 466. (Generic 



relations discussed.) 

 Elkania desiderata (Billings), Ford, 1886, idem, vol. 32, p. 325. (Merely changes generic reference.) 

 Elkania desiderata (Billings), Hall and Clarke, 1892, Eleventh Ann. Kept. State Geologist New York for 1891, PL 

 III, figs. 13 and 14. (No text reference. Figs. 13 and 14 are drawn from casts of the two specimens figured 

 by Billings, 1862d, figs. 62a and 62b, p. 68, the specimens themselves being redrawn in Hall and Clarke, 1892c, 

 PL III, figs. 16 and 17, respectively.) 



