OBOLID.^. 563 



Elkania desiderata (Billings), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, pp. 76 and 

 77, PI. Ill, figs. 15-19. (Part of original description, Billings, 1862d, p. 70, copied on p. 76 and species described 

 and discussed. Figs. 16 and 17 are drawn from the specimens figured by Billings, 1862d, figs. 62a and 62b, 

 p. 68; figs. 15 and 18, which represent casts of the specimens represented by figs. 16 and 17, respectively, are 

 copied from figs. 13 and 14 of the preceding reference.) 



The species has been so thoroughly described by Billings [1862d, p. 69], Ford [1886a, p. 466], 

 and Hall and Clarke [1892c, p. 77], from their respective points of view, that I will confine my 

 observations to the interior characters. As noted under the genus, I regard the cardinal areas 

 as being reversed; that is, sloping within the plane of the margins of the valves instead of with- 

 out, as in Oholus; the space beneath the area and the bottom of the valve is filled with shell, 

 and the margin of the area thus disappears in the anterior slope of the thickened portion of 

 the shell. 



The scheme of muscle scars, as far as they are known, is the same as in Oholus. In the 

 ventral valve the anterior laterals (j) and transmedians (i) occur outside the' main vascular 

 sinuses toward the margin of the valve and in front of the included cardinal area. The space 

 occupied by the central and middle and outside lateral scars is defined on the front slope of the 

 umbonal thickening of the valve. The muscle scars of the dorsal valve include the transme- 

 dian and outside laterals on the outer margin of the main vascular sinuses, and the central 

 and anterior laterals on the margins of the central visceral area. 



The vascular markings include the main vascular sinuses in the two valves, and some 

 inner branches in the ventral valve. 



Elkania desiderata differs from E.amhigua (Walcott), the only related species, in the interior 

 of the dorsal valves, which are the only parts we have for comparison. The shell is also 

 thinner. 



Formation and locality. — Lower Ordovician; (319a) Levis shales, Point Levis, Province of Quebec, Canada. 



Elkania ida (Billings). 



Plate XXX, figures 20, 20a; Plate LI, figures 4, 4a-c. 



Obolella ida Billings, 1862, Geol. Survey Canada, Paleozoic Fossils, vol. 1, p. 71, fig. 63a-b. (Described and dis- 

 cussed as a new species, see below for copy.) 

 Obolella? ida Billings, Walcott, 1886, Bull. U. S. Geol. Survey No. 30, p. 111. (Generic relations mentioned.) 

 BiUingsiaf ida (Billings), Ford, 1886, Am. Jour. Sci., 3d ser., vol. 31, p. 467. (Generic relations mentioned.) 

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



The original description by Billings follows: 



Shell small, obtusely ovate, greatest width at or a little in front of the middle, abruptly narrowed to the beaks, 

 sides and front margin uniformly rounded, the front slightly less convex than the sides. Both valves equally and 

 rather strongly convex, most elevated at about one-third the length from the apex, thence sloping with a gentle curve 

 to the sides and front margin, abruptly descending on each side of the beak and umbones. Ventral valve slightly 

 more pointed above than the dorsal; umbo narrowly convex; beak apparently depressed to the level of the lateral 

 margin. Dorsal valve obtusely angular or narrowly rounded at the beak, the latter not visibly distinct from the 

 cardinal edge; umbo obtusely convex. Surface with a somewhat shining aspect, with very fine concentric striae; 

 a few radiating strife are visible on two of the specimens. Shell apparently very thin. 



Length of a perfect ventral valve, 2 lines; width. If lines. The length and width of the dorsal valve are about 

 equal. 



In some material from the McGill University Museum, kindly lent to me by Sir William 

 Dawson and Dr. Frank D. Adams, I found some fine specimens labeled Obolella ida, collected 

 by J. Eichardson. These specimens show the characters described by Billuigs [1862d, p. 71] 

 and in addition the following: The shell is corneous and built up of many lamellae or layers 

 arranged obliquely to the surface so as to form a thick, strong shell. The interior of the ventral 

 valve has a thickened umbonal portion with a pedicle groove much like that in E. desiderata 

 (Billuigs), but much shorter in proportion; owing to the imperfect condition of the remaining 

 portions, no further details can be determined. The interior of the dorsal valve shows a 

 thickened elevated area like that of the dorsal valve of E. desiderata (PI. LI, fig. Id), but differ- 

 ently shaped. A large transverse depression just in advance of the posterior margin appears 



