FOSSILS OF THE DEVONIAN. 



153 



over that of the dorsal valve. Dorsal valve about as wide as long, strongly 

 convex, and with a well-defined mesial elevation on the lower half of the 

 valve; sinuate in front to unite with the projections of the mesial fold of the 

 opposite valve. 



Surface with from eleven to thirteen simple, angular plications that grad- 

 ually increase in size from the apex; of these there are four on the median 

 elevation and three on the sinus. 



The dimensions of the largest specimen are: Length, 8 mm ; width, 6 mm ; 

 distance through the greatest convexity of the shell, 5.5 mm ; of the smallest 

 specimen: length, 4 mm ; width, 3.75 mm ; convexity, 3 mm . The latter has the 

 plications and the mesial fold and sinus developed as in the larger shell. 

 Other examples are less convex, and with the fold and sinus scarcely defined. 



In its general aspect this species is similar to some forms of the genus 

 Rhynchospira, and with the discovery of more perfect specimens showing 

 the beak and area of the ventral valve, and the interior spires, if they exist, 

 it may possibly be referred to that genus. 



Formation and locality. Lower horizon of the Devonian limestone, 

 Comb's Peak, Eureka District, Nevada. 



Rhynchonella castanea Meek. 

 Plate xv, figs. 1, 1 a, 4, 4 a. 



Khynchonella castanea Meek, 1868. Trans. Chicago Acad. Sci., vol. i, p. 93, pi. xiii, figs. 



Shell subglobose; ventricose on the dorsal side, and somewhat flattened 

 on the ventral; length and breadth subequal or a little longer than wide. 



Ventral valve slightly convex on the umbo, flattened or slightly convex 

 on the sides, and with a more or less well defined sinus that is first seen 

 about the center of the valve. The sinus has from three to six depressed, 

 rounded plications, and varies in form from the subquadrate, linguiform 

 extension, with subparallel sides in the smaller shells, where it is strongly 

 incurved, to the shorter depression outlined by the elevated, acute margins 

 on the lower part that is not curved beneath even in large individuals. Beak 

 abruptly incurved over that of the opposite valve. 



