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



Genus TKIGONAKCA Conrad. 



Trigonarca obliqua Meek. 



PI. xix, Fig. 1; Pi. XX, Figs. 2-6. 



Cucullasa (Trigonarca?) obliqua Meek, 1877, U. S. Geol. Expl. Fortieth Parallel, vol. iv, 

 pt. I, p. 148, PI. 14, Figs. 1, la, b. 



Original description: 



"Shell attaining about a medium size, rhombic- subovate, moderately 

 convex, the greatest convexity being along the posterior umbonal 

 slope, while the anterior ventral region is abruptly cuneate; anterior 

 margin rounded in outline ; base with a shallow semiovate outline, 

 being usually slightly more prominent in advance of the middle ; pos- 

 terior margin long and very obliquely truncated, with a slightly con- 

 vex outline from the hinge to the posterior basal extremity, which is 

 prominent and subangular; hinge margin very short, or little more 

 than equaling one-third the length of the valves; posterior umbonal 

 slope prominent, and more or less angular from the beaks to the poste- 

 rior basal angle; cardinal area short and rather narrow; beaks moder- 

 ately prominent, incurved, and placed one-fourth to one-third the 

 length of the valves from the anterior margin. Surface ornamented 

 by moderately distinct lines of growth, crossed by obscure radiating 

 costse that are wider than the mere linear furrows between. 



"Length, 1.50 inches; height, 1 inch; convexity, about .70 inch." 



This description was based on sandstone casts that retain the sur- 

 face markings to some extent and at the same time show more or less 

 of the internal features. 



The collections from several localities in Huerfano park, Colorado, 

 contain numerous well preserved specimens of this species, which show 

 that the surface is usually marked only by lines of growth, and that 

 the obscure radiating costae above described are on the interior of the 

 shell, and therefore preserved on internal casts. A few of the smaller 

 specimens show obscure radiating lines on the surface, and especially 

 on the umbonal ridge. There is considerable variation in outline, some 

 specimens having almost the form of Ideonarca depressa White. 

 These two species seem to be more closely allied than one would think 

 from the examination of the types alone, but they are probably dis- 

 tinct. The ligamental area is depressed, triangular, with five or six di- 

 varicate cartilage furrows. Hinge line curved as in Pectunculus with 

 subequal transverse teeth, some of which toward the posterior end are 

 slightly larger and curved. Both muscular impressions are bordered 

 by elevated ridges, the posterior one being much the stronger. In all 

 these generic features Trigonarca obliqua agrees with the forms on 

 which Conrad founded the genus, and specifically it is very closely 

 allied to T. triquetra Conrad, from the Cretaceous of North Carolina. 



Locality and position, — The types were collected in Cretaceous sand- 



