582 SYSTEMATIC PALEONTOLOGY 



Superfamily TRIGONIACEA 



Family TRIGONIIDAE 



Genus TRIGONIA Bruguiere 

 [Ency. M6th. Vers., vol. I, 1789, p. xiv] 



Type. Trigonia margaritacea Lam. 



Shell heavy, nacreous within, equivalve, inequilateral, subtrigonal or 

 trapezoidal in outline. Umbones anterior, opisthogyrate, moderately 

 inflated, lunule absent, escutcheon strongly defined; posterior area sharply 

 differentiated by a carina extending from the umbones to the posterior 

 ventral margin ; sculpture upon medial and anterior portions of the disk 

 usually developed and often more or less nodose ; sculpture upon the pos- 

 terior area concentric, radial, divaricate, or absent. Hinge dentition 

 vigorous, two divergent transversely striated cardinal teeth in the right 

 valve, three cardinals in the left, the middle tooth stout, trigonal, medially 

 sulcate, transversely striated, the two outer cardinals compound and rela- 

 tively small, transversely striated within. Ligament groove marginal, 

 opisthodetic, muscle impressions two in number, the posterior the larger, 

 pallial line indistinct, entire. 



Trigonia was one of the major elements during the Mesozoic, the epoch 

 which marks its origin and culmination. Five species still persist in the 

 Australian region, but they are rather distantly connected with the Meso- 

 zoic forms. 



A. Costals not exceeding 16 in number Trigonia eufalensis 



B. Costals exceeding 16 in number. 



1. Shell semi-elliptical in outline, not rostrate posteriorly, costals 



coarser upon the medial portion of the shell than towards the 

 extremities Trigonia cerulea 



2. Shell trigonal in outline, rostrate posteriorly, costals conspicuously 



coarser on the anterior third of the shell, becoming abruptly 

 finer and more regular in arrangement medially. 



Trigonia marionensis 



TKIGONIA EUFALENSIS Gabb 

 Plate XXXIV, Figs. 1, 2 



Trigonia eufalensis Gabb, 1860, Jour. Acad. Nat. Sci., Phila., 2d ser., vol. iv, 



p. 396, pi. Ixviii, fig. 32. 

 Trigonia eufalensis Meek, 1864, Check List Inv. Fossils, N. A., Cret. and 



Jur., p. 9. 



Etymology: T pi, three; ywvia, angles. 



