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BRITISH FOSSIL TRIGONLE. 



Agassiz are from blue Neocomian marls at Neuchatel. D'Orbigny records the occurrence 

 of his T. rudis at Ventoux (Vauclose) ; Morteau (Doubs) ; Saint Souveure (Yonne). 



Tkigonia spectabiijs, Sow. Plate XXVI, figs. 1, 2, 3, 4. 



Trigonia spectabilis, Sow. Min. Conch., vol. vi, tab. 544, p. 83, 1829. 



— — Pusch. Polens Palaontol., p. 60, 1837. 



— — Agassiz. Trigonies, p. 8, 1840. 



— — Morris. Catal., p. 229, 1854. 



— nodosa, Pictet et Renevier. Gres Vert, p. 484, pi. xxxv, fig. 5, 1857. 



Shell subquadrate, short, depressed in the young state, thick and moderately convex 

 when fully developed ; umbones small, pointed, scarcely elevated above the superior 

 border, of which they form the anteal extremity ; the anterior border is truncated, it 

 descends almost perpendicularly, but is curved at its junction with the lower border; 

 hinge-border straight, forming nearly a right angle both with the anterior border and 

 with the lengthened siphonal border. The surface of the area is equal to three sevenths 

 of the entire valve; it is slightly convex, but depressed at the well-marked mesial 

 junction of the two portions • the superior or more depressed half has a few unequal and 

 imperfect rows of small nodes, or, in other instances, the few nodes are scattered irre- 

 gularly ; the other or inner portion of the area has at its umbonal extremity several 

 subangular transverse ridges, which are continuations of those upon the other portion of 

 the valve ; each of these forms a prominent node at the position of tiie marginal and 

 also of the median carina to the number of four or five rows, posteally to which the area 

 has only a few irregular and obscure nodosities, which near to the siphonal border are 

 effaced by plications of growth. The escutcheon is lengthened, very narrow, flattened, 

 and inconspicuous ; it is slightly overwrapped at its outer border by the nodosities of 

 the inner carina of the area which are extended upon the escutcheon. The other 

 portion of the shell has, mesially, four or five rows of very large, depressed, rounded 

 varices ; they become small and curved near to the pallial border, and enlarge con- 

 siderably towards the angle of the valve ; each row has about eight or nine large, de- 

 pressed, ovate, and scarcely separated nodes, the longer diameter of these is across 

 the varices ; or the nodes are sometimes only obscurely defined or partially united 

 in the rows j the first-formed four or five varices are entire, narrow, angulated, and 

 transverse ; their anteal extremities are bent upwards suddenly and perpendicularly ; 

 there is also an additional short varix adjoining the anteal extremity of the fourth row. 

 The two last-formed rows of pallial varices are comparatively small, depressed, and cord- 

 like ; their general direction coincides with the lines of growth. In fully developed 



