FOSSILS OF THE DEVONIAN. 205 



tral dorsal region of the valve, terminating about three-fifths the distance 

 across the valve from the dorsal margin; the posterior sulcus is narrow and 

 curves backward, while the anterior is quite broad, with a forward curve, 

 each having its origin on the dorsal margin as a faint groove and deepening, 

 with the convexity of the valve ; this, united with the opposite curvature 

 of the sulci, forms an elongate lobe or tubercle on the central dorsal portion 

 of the valve ; on casts of the carapace, and on valves where the two sulci 

 are united by a slight sulcus at their lower termination, the size and appear- 

 ance of the tubercle is greatly increased; a shallow groove occurs just within 

 the anterior and posterior dorsal angles and extends a short distance down 

 the anterior and posterior margins, where it becomes obsolete; two small, 

 elongate nodes are situated near the dorsal margin, each a little within the 

 anterior and posterior dorsal groove respectively ; the surface of the nodes, 

 the central lobe, and margins of the sulci are smoothly and evenly rounded. 



Surface finely granulated, the granules arranged along minute, irreg- 

 ular striae, or lines that radiate from the outer margins of the sulci to the 

 anterior, posterior, and ventral margins. 



The presence of the two sulci, as seen in all the adult individuals, 

 properly refers this species to the genus Beyrichia. A study of a series of 

 specimens, however, that pass from the largest size down to individuals 

 l mm in length, shows that the posterior sulcus becomes more obscure with 

 the decrease in size of the individual, and that when the shell is l mm in length 

 the posterior sulcus is obsolete and a true unisulcate Beyrichia results that 

 may be referred to the genus Primitia (plate xvii, fig. 4 a). The adult is a 

 member of the corrugate group of Beyrichise, of which Beyrichia Wilckensiana 

 Jones (Ann. and Mag. Nat. Hist, 2d ser., vol.xvi, p. 83, pi. v, figs. 17, 18, 1855) 

 is the type, and the young form, of the "Sirnplices" of which B. (Primitia) 

 strangulata (Salter) Jones (Ann and Mag. Nat. Hist, 2d ser., vol. xvi, p. 17 1, 

 pi. vi, fig. 18, 1855; Ibid., 3d ser., vol. xvi, p. 416, 1865), or B. (Primitia) 

 simplex Jones (Quart. Jour. Geol. Soc., vol. ix, p. 161, pi. vii, fig. 7) is the type. 



Formation and localities. Lower horizon of the Devonian limestone; 

 Newark Mountain, Eureka District, and Lone Mountain, 18 miles north- 

 west of Eureka, Nevada. 



