﻿9S 



BRITISH FOSSIL TRIGONLE. 



Trigonia Lingonensis, Bum. Plate XXII, figs. 1, 1 a, 2, 3, 4. 



Trigonia Lingonensis, Dumortier. Etudes Jurrassiques du Rhone, p. 2/5, pi. xxii, 



figs. 6—8, 1861. 



— — Tate. Discovery of the oldest known Trigonia in Britain, 



< Geol. Mag.,' vol. ix, No. 97, p. 306, 1872. 



Shell ovately trigonal, very convex ; umbo antero-mesial, pointed, much produced, 

 and slightly recurved ; anteal, lower, and posteal borders curved elliptically ; hinge- 

 border nearly straight, sloping obliquely downwards, and forming an obtuse angle with 

 the rounded siphonal border of the area. Escutcheon wide, depressed, traversed 

 by transversely oblique, delicate plications, which become conspicuous and more 

 strongly marked upon the obtuse inner carina. Area concave, bounded by two 

 raised, obtusely rounded prominences or carinas, traversed mesially for about a 

 moiety of its length by a small furrow ; both the area and its carinas are 

 traversed obliquely by very numerous unequal rugose elevations ; near to the 

 umbo these form a row of minutely knotted papillae upon the border of the narrow 

 ridge-like, inner carina. The marginal carina is distinct and elevated only as 

 compared with the area, but has no distinctiveness or separation when compared with 

 the other portion of the shell whose rugae pass across it without interruption ; near to the 

 apex, however, it becomes elevated, narrow, and ridge-like, and the surface anterior to it 

 has densely arranged acute rugae. The other portion of the surface has a numerous 

 irregular and unequal series of rug?e, which take the direction of the lines of growth ; all 

 originate at the pedal border as narrow, densely arranged plications, which become less 

 conspicuous and nearly evanescent upon the middle of the valve ; they are continued unin- 

 terruptedly across the area and escutcheon. There are also, in some instances, several 

 longitudinal sulcations which are conformable with the rugae in their direction, and are 

 similarly unequal in their distinctness and distances ; other specimens are nearly destitute 

 of these sulcations. The area forms nearly a right angle with the other or pallial portion 

 of the valve, so that, when a specimen is placed in a horizontal position and viewed from 

 above, the area and escutcheon are scarcely visible. 



Dimensions. — The largest of the specimens herewith figured has the length, measured 

 from the apex to the posteal extremity, 28 lines ; from the upper extremity of the 

 siphonal border across the valve, at right angles to the length, 24 lines; thickness 

 through the single valve 9| lines, length of the siphonal border 10 lines, length of the 

 superior border of the escutcheon 18 lines. 



These dimensions do not refer to the largest specimen obtained, as they are exceeded 

 by one in the museum of the Philosophical Society at York ; very rarely also the valves 

 are found in position. 



