228 PALEONTOLOGY OF THE EUEEKA DISTJRIOT. 



Surface marked by from 25 to 30 simple radiating costae, the inter- 

 spaces wider than the costse, and with intercalated smaller costse rising in 

 some of them; concentric striae and lines of growth are more or less dis- 

 tinctly shown. Right valve unknown. 



The interior of the hinge unknown. 



Dimensions: height and width of average size specimen, 3.5 cm ; con- 

 vexity of left valve, .75 cm . 



This species shows considerable similarity to A. Eurekensis. It differs 

 in the presence of a larger number of costse and in its more orbicular form. 



Formation and locality. Lower portion of Lower Carboniferous lime- 

 stone in canon directly south of 'a conical hill on east side of Secret-canon- 

 road Canon, Eureka District, Nevada. 



Aviculopecten Piiitoensis, n. sp. 

 Plate viii, fig. 6. 



Shell small, rounded ovate in outline exclusive of the wings\ Left 

 valve gently convex. Hinge-line about two-thirds as long as the greatest 

 width of the shell below. Ears subequal in size, the anterior with a some- 

 what more deeply cut, rounded sinus beneath. Beak small, obtusely 

 pointed and scarcely extending beyond the cardinal margin. 



Surface marked by numerous fine radiating linear costae, twenty in 

 a distance of five millimeters, with a few finer striae or costae in some of the 

 interspaces; the costae are present on the ears and are strongly defined on 

 the anterior ear; a few concentric lines of growth appear on the posterior 

 ear and extend on to the disk of the shell as rather indefinite undulations 

 or lines of growth. 



Interior characters unknown. 



Dimensions: height, 2 cm ; width, 2 cm ; convexity of left valve, 2 mm . 



This pretty little shell is of the type of Aviculopecten Coxanus M. and 

 W. (Geol. Surv. 111., vol. ii, plate xxvi, fig. 6 , &), differing only in its more 

 erect form and shorter ears. In form" it also approaches A. (Pecteii) plicatus 

 Sowerby, as figured by De Koninck (Anm. Foss., plate iv, fig. 6). 



Formation and locality. Lower portion of Carboniferous Group, in 

 canon north of Pinto Peak, Eureka District, Nevada. 



