SrANTON.] ARCIDiE. 89 



they are abruptly flexed forward, and blend with the "ventral border. 

 Grossing the lines of growth upon the surface above the mesial angle, 

 there are coarse but indistinct radiating strire, and occasionally still 

 more indistinct traces of similar ones below that angle, all of which 

 are more discernible upon the anterior than upon the posterior part of 

 the shell. More or less of the test is preserved upon all the specimens ; 

 but no trace of the pearly layer has been detected, all the test having 

 the usual prismatic structure. 



" Some of the largest examples measure 7£ CTn in width along the poste- 

 rior margin, and they must have been not less than 17 cm in length when 

 entire. The large size, proportionally great width, and angular aspect 

 of this shell distinguish it from any other likely to be confounded with 

 it. It resembles P. renauxiana d'Orbigny, as figured by him in Pal£on- 

 tologie Francaise, but it expands much more rapidly in width than that 

 species does, is not proportionally so thick, and has a very different 

 posterior marginal outline. In that shell it is the middle of the poste- 

 rior margin that is most extended, while in ours the greatest extension 

 is much below the middle. 



"Position and locality. — Strata of the Cretaceous period; east of 

 Mount Taylor, 1 mile south of Pajuate, New Mexico." 



The type of Pinna stevensoni, which was found near Fort Win gate, 

 New Mexico, only a few miles from the typical locality of P. petrina, 

 seems to be only an unusually large specimen of the latter species. 

 The absence of the anterior portion, which is always more angular and 

 often bears radiating lines, gives this specimen an unusual aspect. 



The exact position of the beds from which the types came is not 

 known. The species is not uncommon in the Pugnellus sandstone of 

 Huerfano park, Colorado, and a few examples were found in the Ben- 

 ton shales on the Arkansas river, 20 miles above Pueblo, Colorado. A 

 Pinna occurs in the sandstone of the " second ridge " at Coalville, 

 Utah, that may be this species. 



ARCID^E. 



Genus BAKBATIA Gray. 



Barbatia micronema Meek (sp.). 



PI. xxi, Figs. 1-4. 



Trapezium micronema Meek. 1873, Ann. Rept. U. S. Geo!. Sur. Terr, for 1872, p. 493.— 

 White, 1879, Ann. Rept. U. S. Geol. Sur. Terr, for 1877, p. 293, PI. 10, Fig. 5. 



Area? coalvillensis White, 1876, Geol. Uinta Mts., p. 115. 



Barbatia coalvillensis White, 1879, Ann. Rept. U. S. Geol. Snr. Terr, for 1877, p. 286, 

 PI. 6, Figs. 2a and b. 



The type of this species is a cast of both valves on which the surface 

 markings are partially preserved, but it is broken and distorted by 

 pressure, so that the generic features are obscured and the outline is 

 not complete. This specimen is from Cretaceous coal-bearing sand- 



