BIVALVIA. ol 
posterior extremities, where they do not form large, continuous varices, bent upwards at 
a considerable angle, as in the latter species. 7" costatula, Lyc., is more convex, the 
cost are more regular, smooth and concentric, the area also is much larger, which imparts 
a subquadrate figure to the outline; other species are more remotely allied. 
Geological Position and Locality. The slate of Collyweston, Northamptonshire, in 
which the specimens are usually compressed. 
Triconia ciyTHia, D’Orb. Suppl., p. 48, Tab. XXXVI, fig. 2; Tab. XL, fig. 5. 
Some fine specimens received subsequently to the printing of page 48 have enabled 
the artist to illustrate the more adult aspect of this species. Tab. XL, fig. 5 @ exhibits 
the nodulous character of the posterior extremities of the cost, their anterior portions 
remaining regular and concentric; fig. 5 is an aged specimen, exhibiting further changes. 
In common with many other of the Jurassic Trigoniz in the ultimate stage of growth, the 
smooth costz are no longer regular or concentric; they become less distinctly marked, 
broken, undulating or wrinkled, constituting the approach to the period when all orna- 
mentation ceases. 
Triconia TRIPARTITA, Forbes. Tab. XL, fig. 4. 
TRIGONIA TRIPARTITA, Forbes. Journ. Geol. Soc., vii, tab. 5, fig. 11. 
- — Morris. Catal., 1854, p. 229. 
Testa ovato-trigona, subdepressa, umbonibus obtusis sed recurvatis, latere antico rotunda, 
postico subconcavo oblique declivi, antice costis levigatis parvis obliquis crebris, postice 
aliis (7—8) obliquis magnis depressis, nodulatis ; area subconcava, sulco mediano obliquo, 
costis transversalibus penes apicem instructis. 
Shell ovately trigonal, rather depressed ; umbones obtuse, but recurved ; anterior margin 
rounded ; posterior margin somewhat concave, sloping obliquely downwards ; the anterior 
side has numerous (about thirty) delicate, oblique, smooth cost, which are interrupted 
posteally by others which cross them nearly at right angles ; the latter costee (about seven 
or eight) are large, nodulous and depressed, the two latter only reach the lower border ; 
the marginal carina is but little conspicuous ; the area is somewhat concave, it is transversed 
by a mesial furrow, and has a few transverse coste near to the apex. Our specimen is 
slightly imperfect at the apex and at the inferior border. 
Geological Positions and Localities. A single example from the Cornbrash of Chippen- 
ham, in the collection of W. Walton, Esq. ‘This pretty species was also obtained by the 
late Professor E. Forbes in a stratum of yellowish, crumbly limestone and shale, beneath 
the Oxford Clay at Lock Staffin, in the Isle of Skye, associated with fresh-water and marine 
