BIVALVIA. 51 



posterior extremities, where they do not form large, continuous varices, bent upwards at 

 a considerable angle, as in the latter species. T. costatula, Lye., is more convex, the 

 costae 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. 



Geoloffical Position and Locality. The slate of Collyweston, Northamptonshire, in 

 which the specimens are usually compressed. 



TRIGONIA CLYTHIA, D'Orb. Suppl., p. 48, Tab. XXXVII, 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 a exhibits 

 the nodulous character of the posterior extremities of the costa3, their anterior portions 

 remaining regular and concentric ; fig. 5 is an aged specimen, exhibiting further changes. 

 In common with many other of the Jurassic Trigonise in the ultimate stage of growth, the 

 smooth costae 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. 



TRIGONIA TRIPARTITA, Forbes. Tab. XL, fig. 4. 



TRIGONIA TRIPARTITA, Forbes. Journ. Geol. Soc., vii, tab. 5, fig. 11. 

 Morris. Catal., 1854, p. 229. 



Testa ovato-triffona, subdepressa, umbonibus obtusis sed recurvatis, lalere antico rotunda, 

 postico subconcavo oblique dcclivi, antice costis laevigatis parvis obliquis crebris, postice 

 aliis (7 8) obliquis magnis dcpressis, 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 costse, which are interrupted 

 posteally by others which cross them nearly at right angles ; the latter costae (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 costae 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 



