74 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



robust, having proportionally larger wings, narrower body, and a more 

 nearly erect axis." 



All the types are in the form of casts in sandstone and more or 

 less distorted by pressure. In Huerfano park, Colorado, specimens 

 were found that agree very well in form with the types, but some of 

 them are very much larger. These retain portions of the shell, show- 

 ing that it was unusually thick, especially iu the umbonal region, and 

 that the surface is marked by fine, closely arranged, concentric lines. 

 Associated with these in the same bed there are other, smaller indi- 

 viduals that are much more oblique, some of which are scarcely dis- 

 tinguishable from some specimens of Avicula linguiformis. (See PI. ix, 

 Figs. 9 and 10.) But such specimens as that represented by Fig. 8 of 

 the same plate are so nearly intermediate between the two extremes 

 that I prefer to keep them for the present under the one name. Avicula 

 caudigera Zittel, from the Cretaceous of Gosau, is a closely related 

 species. 



The largest specimen seen, a left valve, measures 73 mm in height and 

 61 mm in length just below the wings; greatest convexity of the single 

 valve about 15 mm . 



Locality and position. — The type came from the " second ridge n at 

 Coalville, Utah, where it is associated with Gervillia propleura, Pugnel- 

 lus fusiformiSj etc. ; the types of Pteria erecta were found at probably 

 about the same horizon in Lower Potato valley, southern Utah, and 

 the specimens now figured come from the Pugnellus sandstone in Poi- 

 son canyon and on Williams creek, Huerfano park, Colorado. 



Genus GEEVILLIA Defrance. 



Gervillia propleura Meek (sp.). 



PI. x, Figs. 1, 2, and 3. 



Avicula (Pseudoptera) propleura Meek, 1873, Ann. Rept. U. S. Geol. Sur. Terr, for 1872, 



p. 489. 

 Avicula (Pseudoptera) rhyiophora Meek, 1873, ibid., p. 490. 

 Pteria (Pseudoptera) propleura White, 1879, Ann. Rept. U. S. Geol. Sur. Terr, for 1877, 



p. 281, PI. 10, Figs. 2a, b, c. 

 Compare Gervillia gregaria Shuinard, 1860, Trans. St. Louis Acad. Sci., vol. 1, p. 606. 



Shell very inequivalve, more or less oblique, varying in outline from 

 obliquely ovate- sub trigonal to rhombic-suboblong, nearly twice as high 

 as wide; left valve moderately convex, with the greatest convexity 

 toward the anterior side just below the beaks on the umbonal slope, 

 wmich is here somewhat angular, but becomes broad and flat towards 

 the basal margin; beak small, pointed, almost terminal and projecting 

 but little beyond the hinge line; the small area in front of the beak 

 rather abruptly depressed, forming a more or less distinct anterior 

 auricle; posterior alation represented by a broad triangular flattened 

 area, not distinctly defined from the body of the shell; hinge-line 



