cakbonh-'eeous period. 151 



Family PINNID^. 



Genus PINNA Liunaeus, .1758. 

 Pinna peracuta Sbnmard (?). 



Plate XI, fig. .5 a. 



From the Mesa edge, near Eelief Spring, Arizona, a single specimen of 

 Pinna was obtained, Avhicli is too imperfect for Tull specific determination. 

 It has the aspect and general features of a young example of P. peracuta 

 Shumard, to which species it probably belongs. Certain traces of lines of 

 growth, howevei', indicate that our specimen was much more slender than 

 the typical forms of that species, and also that the border below .the 

 middle sloped much farther forward than it is known to do in P. peracuta. 

 The species associated with it are the associates of P. peracuta elsewhere, 

 which adds force to the supposition that our specimen belongs to that species. 



Examples of P. peracuta are not uncommon in the Carboniferous strata 

 of the States bordering the Mississippi ; but, although the collections con- 

 tain a greater number of species from the strata of the Carboniferous period 

 than any other, this is the only example of Pinna found among all the 



Paleozoic fossils. 



Family PTERIID.^. 



Genus MONOPTERIA Meek and Worthen, 1866. 



Monopteria Marian White. 



Plate XI, fig. 4 a, b, and c. 



Monopteria Marian White, 1874, Expl. Surv. west lOOfch Merid., Prelim. Rep. Invert. 

 Foss., 22. 



Shell of moderate size, slender, nearly or quite equivalve, narrow and 

 much extended posteriorly, the curvature being much the greater in the 

 anterior half of the shell, the posterior half being nearly straight ; body of 

 the shell gradually tapering to near the posterior end, which is abruptly 

 rounded ; a more or less prominent ridge which is sometimes in part 

 raised as a distinct carina, extends along the middle of the body of each 

 valve from the beak to the posterior end ; from this carina, or angle, the sides 

 slope abruptly to both the inferior and upper borders, so that a cross-section 



