82 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Family TRIGONUD^. (See page 57.) 

 Genus TRIGONIA, Bruguieke. 



Synon.—Trigonia, Bedo. Encyc. Meth. I, 1789, xiv, pi. 237.— Lamk. Prodr. 1799, SC ; Id. Syst. An. 1801, 116, and 

 An. du Mus. IV, 1804, p. 351.— Roissy, Mol. VI, 1805, 392, &c. 



Lyrodon, G. B. SowEKBT, Genera Shells, 1833, faso. 41. 



Lirodon, BuoxN, Leth. 1837 (sec. ed.) 367 and 700. 



Lyriodon (part), Goldf. Petref. Germ. II, 1837, 196.— Bronn, Jahrb. 1838, p. 108. 

 Etym. — Tfi'yaiH'c, three cornered. 

 Type, — Trigonia scabra, Lamk. 



Shell subtrigonal, longitudinally ovate, elongate, or subcircular ; postero-dorsal 

 region often provided with a more or less distinctly defined escutcheon or corselet. 

 Surface ornamented with radiating, oblique, or concentric costa3 or rows of nodes; 

 rarely smooth. Beaks usually elevated. Ligament short and prominent. Hinge 

 thick, composed of two large diverging, elongate, transversely furrowed teeth, in 

 the right valve, and three or four in the left, furrowed only on one side. Im- 

 pressions of adductor muscles usually well defined, the anterior being located near 

 the beaks. Scar of posterior pedal muscle located a little above, and in front 

 of, the impression of the posterior adductor; antero-pedal scar generally placed 

 within the cavity of the beaks, sometimes wanting in the right valve. 



Many of the extinct species generally referred to this genus diifer so widely in 

 form and surface ornamentation from each other, as well as from the living Trigonias, 

 as to leave room for doubts whether or not they really all belong to one and the same 

 genus. These diff"erences attracted the attention of Prof. Agassiz, who separated 

 the several tj'pes into distinct sections, which he accurately describes in his excellent 

 monograph of the genus. These sections may be briefly characterized as follows : — 



1. " L,e9 Scaplioides," Agassiz. 



Shell subtrigonal, longer than high ; anterior side short and truncated ; posterior side long and very abruptly 

 rounded or subangular at the extremity. Dorsal corselet large and nearly smooth, excepting the lines of 

 growth ; not defined by a ridge or sulcus on either side. Surface ornamented with varices in front, and 

 more or less nodose, transverse or oblique costoe on the flanks. 



Type. — Trigonia navis, Lamk. (^Jurassic and Cretaceous.') 



2. "L-es Clavellees," Agassiz. 



Shell more rounded in front than in the last group ; corselet well developed and often bounded by a ridge on 

 either side, sometimes with strong marks of growth. Surface ornamented with more or less nodose costse, 

 which pass from the margins of the corselet obliquely downwards and forwards, often becoming broken up 

 into a series of isolated tubercles on the sides and front of the valves. 



Example. — T. clavellata, SowERBY. (Mainly Jurassic.) 



3. "L-es Carrces," Agassiz. 



Shell shorter and more truncated at each extremity than the last ; also with less regular costae on the flanks, 



and a larger and more compressed corselet. Marks of growth often distinct on the corselet. 

 Example, — T. quadrata, Ag. {Upper Jurassic and Cretaceous.) 



4. "E-es Scalires," Agassiz. 



Shell rounded and gibbous in front, elongate, narrowed and subrostrate behind. Corselet separated from the 

 flanks by a more or less distinct groove ; ornamented with transverse costne. Flanks and front with tuber- 

 culose, or subspinous costse, which pass from the margins of the corselet to the base and front, those on 

 the anterior part of the valves curving forward. 



Example. — T. aliformis, Sowerby. (^Mainly Cretaceous.) 



