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 7° 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 Radipole 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, Zyc. Tab. XL, fig. 6. 
TRIGONIA TUBERCULOSA, Lycett. Ann. and Mag. Nat. Hist., 1850, p. 12, t. 11, fig. 9. 
a —- Morris. Cat. Brit. Foss., 1854, p. 229. 
Testa ovato-trigona, subdepressa, umbonibus parvis, recurvatis, marginé anterioré et 
inferioré rotundo, marginé posticd eacavatd, area augustata, transverse plicata, plicis magnis 
acutis ; 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 
carinz, and with transverse acute plications, every second plication forming a varix upon 
the inner carina; the marginal carina is delicately tuberculated; the post ligamental 
lanceolate space is small, with several oblique varices ; the sides of the 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 costz ; 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 
costee may be counted in a specimen whose length is only nine lines. 
Geological Positions and 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 
