46 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



Shell small, inflated, short, subtrigonal ; umbones depressed, mesial, obtuse and incurved; 

 the anterior border is rounded, sloping downwards and uniting with elliptical curvature of 

 the lower border ; the posterior side slopes abruptly downwards, it has an oblique posterior 

 carina, which becomes conspicuous and raised towards its lower extremity ; it separates a 

 posterior, depressed, lanceolate area from the sides of the shell. The surface has obscure 

 longitudinal striations, and several folds of growth. 



The inflated figure, short posterior side and projecting oblique posterior carina, dis- 

 tinguish it from Leda lachri/ma, Sow., and also from other species of the Lower Oolites. 



Geological Positions and Localities. The Cornbrash of Scarborough, in which it occurs 

 rarely. Professor Phillips records it in the lower stage of the Liferior Oolite (Dogger), 

 and in the gray limestone or upper stage of the same formation upon the coast of Yorkshire. 



Trigonia elongata, Sow. Tab. XXXIX, figs. 6, 6 a. 



Trigonia elongata, Sow. Min. Con., t. 431. 



— — UOrb. Prodr., vol. i, p. 338. 

 -^ — Morris. Catal, 1854, p. 228. 



— — Oppel. Juraformation, p. 525. 



— — Damon. Geol. Weymouth, Suppl., pi. 2, figs. 1, 2. 



Testa suhtrigona, alia, convexa, antici brevissima truncata, costis, magnis, subhmizon- 

 talibuSi leviter undulatis ; umbonibus prominentibus aaifis incurvis ; area cardinali lata, 

 ornatissima, distincte tripartita, carinis prominentibus, de?iticulatis. 



Shell subtrigonal, very convex and lengthened ; anterior side short, its border abruptly 

 truncated with numerous large, nearly horizontal and slightly undulated costae ; the 

 umbones are elevated and much incurved ; the posterior area (which nearly equals in size 

 the other portion of the surface) is very wide, and is separated into three distinct parts by 

 as many prominent denticulated caringe ; the marginal carina is very large and nearly 

 straight ; the mesial and inner carinse though smaller are likewise conspicuous in both the 

 valves ; the space between the mesial and inner carina is much depressed and its orna- 

 mentation is very delicate ; the superior or post ligamental space is short and wide, it has a 

 few elevated perpendicular plications. The convexity of the united valves is somewhat 

 greater than the breadth of the shell, and equal to two thirds of the length of the marginal 

 carina. 



The general figure and other characters are so strongly defined that it will not readily 

 be mistaken for any other example of the group of the Costata ; the figure of the Cornbrash 

 specimens agrees with those from the Oxford Clay, but the sculpture upon the area is less 

 strongly marked in specimens from the latter formation, which are also usually smaller. 

 Compared with other examples of the same group of species, T. elongata is remarkable 

 for the short, widely-separated horizontal costae, for the great size and straightness of the 



