BIVALVIA. 47 



marginal carina, for the prominence of the sculpture upon the tripartite area, and more 

 especially for the shortness and great breadth of the superior or post-ligamental space, 

 which, when the valves are united, becomes cordate rather than lanceolate. D'Orbigny 

 (Prodrome) believes it to be identical with T. cardissa, Agassiz, it is, however, only necessary 

 to compare the marginal carina in the two forms to perceive their distinctness. 



Geological Positions and Localities. It is abundant in the Oxford Clay of the southern 

 counties, more especially at Radipolc near Weymouth, and in the Cornbrash of the coast 

 of Yorkshire, at Gristhorp, and at Scarborough. The foreign localities cited are France, 

 Dives, Villers (Calvados), Clucy, Mont Orient, near to Salins (Jura), Montsec, near to St. 

 Mihiel (Meuse), Marault, near to Chaumont (H. Marne), Beaumont, Pizieux, Chauffour 

 (Sarthe). 



Trigonia tuberculosa. Lye. Tab. XL, fig. 6. 



Trigonia tuberculosa, Lycett. Ann. and Mag. Nat. Hist., 1850, p. 12, t. II, fig. 9. 

 — — Morris. Cat. Brit. Foss., 1854, p. 229. 



Testa ovato-friffona, subdepressa, umhonihus parvis, recurvatis, margine anteriore et 

 inferiore rotundo, margine postico excavato, area august ata, transverse plicata, plicis magnis 

 aciitis ; carina marginali delicati nodulois ; carina interna varicibus magnis regularibus 

 ornata ; area lanceolata varicibus paucis obliquis ; valvis lateribus costis numerosis concen- 

 tricis et dense tuberculosis, tuberculis crebris elevatis, compressis. 



Shell ovately trigonal, depressed ; umbones small, mesial and recurved, anterior and 

 lower borders rounded, superior border rather excavated ; area narrow, with two oblique 

 carinse, and with transverse acute plications, every second plication forming a varix upon 

 the inner carina; the marginal carina is delicately tuberculated ; the post liganiental 

 lanceolate space is small, with several oblique varices; the sides of tlie valves have very 

 numerous, closely arranged, concentric tuberculated, costa ; the tubercles are much elevated, 

 and compressed laterally, imparting to them a club-shaped figure, the lower extremity of 

 each extending to the succeeding costa. 



A pretty little species, remarkable for the delicacy and salient features of its orna- 

 mentation. The characters of the tubercles upon the sides of the valves closely resemble 

 those in Trigonia elathrata Ag., but in other particulars the two species are widely 

 separated ; the close contiguity of the extremities of the tubercles between row and row 

 gives to them, when viewed from the posterior side, the appearance of forming a series of 

 vertical costse ; the tubercles are, however, very well separated in the rows, and towards 

 the lower border they project considerably from the sides of the valves ; eighteen rows of 

 costse may be counted in a specimen whose length is only nine lines. 



Geological Positions arid Localities. The specimen figured is from the cabinet of 

 the Rev. A. W. Griesbach, and was obtained by him from the Cornbrash of Rushden ; it 



