TRIASSIC FOSSILS. 31 



together with the resemblance of the outline, taken from the striae of growth, to 

 Halobia Lomcllli of the Saint Cassian formation, renders it probable that they are 

 generically related, especially when we take into consideration their geological 

 relation. 



Fig. a, represents the inside of a fragment of a valve, the largest specimen col- 

 lected. Fig. b, is from the surface of one of the most perfect specimens, c, is a 

 magnified view of the ribs. 



Localities : Star Canon, Bucna Vista District, Nevada Territory, and GifFord's 

 Eanch, Plumas County, California. 



MONOTIS, Bronn. 



M. SUBCIRCULAKIS, 11. 8. 

 PI. 6, Figs. 29, 29 a. 



Shell flattened, subcircular, produced behind; beaks placed 

 between the middle and anterior end ; anterior, basal and pos- 

 terior margins forming nearly a perfect semicircle ; the anterior 

 margin curving regularly inwards, above, to the beaks. Wing 

 short, broad, and obliquely truncated behind. Surface marked 

 by from forty to fifty irregular, radiating ribs, alternating in size, 

 and crossed by fine, squamose, concentric lines. 



Figures, natural size, and a magnified view of the surface. 



Locality: Gifford's Eanch, Plumas County, California; collected by Prof. Brewer; 

 and Star District, Humboldt County, Nevada Territory, by Mr. Homfray. 



This shell is very closely allied to M. salinaria, Bronn ; but differs from that 

 species, as figured by Goldfuss, in being less oblique, more prominent below, less 

 produced behind ; in the anterior end being regularly rounded above, instead of 

 bearing a small angle, and in the ear being shorter and broader. The number of 

 ribs is about the same, and they resemble each other in their being irregularly 

 undulated and alternating, as well as in the cross stria?. The most obvious differ- 

 ence is in the rounded upper end of the anterior margin of this species, instead of 

 the very distinct angle of M. salinaria, almost amounting to a second ear. 



RHYNCHOPTERUS, N. Gen. 



Aviculoid, hinge straight, no area; with a small acute ear 

 anteriorly, posterior side not alate, rounding upwards and inward 

 to the cardinal margin. 



