484 



NORTH AMERICAN INDEX FOSSILS. 



Beaks nearly terminal anteriorly, directed posteriorly. Surface 

 bearing rows of tubercles or radiating or concentric ribs. Cardi- 

 nal teeth two in the right valve ; three in the left. Ligament ex- 

 ternal. Pallial line simple. Jurassic— Recent. 

 326. T. quadrangularis Hall & Whitf. (Fig. 646.) Jurassic. 



Trigonal to subquadrangular, with two 

 sets of ridges making an acute angle with 

 each other. 



Upper Jurassic of Wyoming, Dakota, 

 etc. 



327. T. taffi Cragin. Comanchic. 



Very large (length and height each 

 Fig. 646. Trigonia quad- over 3 inches), rounded-subquadrate. 

 rangularis. (After Hal] and Posterior dorsal area ornamented with 

 Whitfield.) numerous, parallel, oblique ridges sepa- 



rated by much broader interspaces ; the ridges near the beak be- 

 come resolved into series of small granules. The rest of the sur- 

 face covered with close rows of small tubercles. 



Trinity of Texas. 

 328. T. emoryi Conrad. (Fig. 647.) Comanchic. 



Ribs 30 or more, strong and tubercled, becoming indefinite pos- 



Fig. 647. Trigonia emoryi. (After Shattuck. ) 



teriorly ; they change direction in passing over the posterior dorsal 

 area and again over the escutcheon. 



Fredricksburg and Washita : Texas, Mexico, Oklahoma. 

 329. T. equistriata Gabb. Comanchic-Cretacic. 



Small, trigonal, elongate. Beaks prominent. • Cardinal margin 



