70 BRITISH FOSSIL TRIGONI^E. 



less posteal curvature towards the carina ; the tubercles also appear small for a specimen 

 of such advanced growth, differences which indicate a variety. 



Stratigrapliical position and Locality. Our specimen was obtained in the Trigonia- 

 bed of the Coralline Oolite at Pickering; no second specimen has come under my 

 observation. 



Trigonia compta, Lye. Plate XV, figs. 5, 6, 7. 



Trigonia compta, Lycett. Suppl. Mon. Gr. Ool. MoUusca, Pal. Soc, 1863, p. 50, 

 pi. xl, fig. 1. 



Shell ovately trigonal, somewhat depressed ; umbones moderately elevated, antero- 

 mesial, not recurved, obtuse ; anterior side produced, curved elliptically with the lower 

 border ; posterior border truncated at its extremity. Escutcheon narrow and incon- 

 spicuous. Area rather wide, flattened, with a mesial furrow and three small tuberculated 

 carinas ; its general surface is smooth, with faintly traced lines "of growth. In other 

 specimens the carinas are evanescent, and the area is altogether smooth ; its entire surface 

 is raised, so that the marginal carina, although so little conspicuous, forms a prominent 

 ridge compared with the more depressed, adjacent costated portion of the valve. . 



The other portion of the surface has about twelve rows of costas, which pass from the 

 anterior border obliquely downwards ; they are very narrow, elevated, and sub-tuberculated, 

 or occasionally plain ; posteally they increase in size, become partially disunited, and 

 form two distinct, large, depressed nodes or varices, which curve upwards and meet the 

 marginal carina at a right angle ; each row of varices corresponds to every alternate row 

 of the narrow anteal costas. 



The general ornamentation has but little prominence, and not unfrequently it is rather 

 obscure. 



Young specimens, when only three hues across, have narrow, horizontal, plain costas, 

 which are slightly curved upwards at their posteal extremities ; the marginal carina is then 

 prominent. 



T. compta is a small species not uncommon in the slate of Collyweston ; the specimens 

 are usually more or less compressed ; it also occurs more rarely in the sand of 

 Northampton, in which it retains its original convexity. It differs from 2\ Moretoni in 

 having much fewer costae, which do not form continuous curves as in that species, but 

 are disunited posteally, forming a less numerous series of short large varices ; the area 

 destitute of large transverse rugose folds, is another distinctive feature. 



From T. costatula it is separated by the more lengthened form, less convexity, larger 

 area, and the presence of the short, curved, large posteal varices. The same general 

 features, together with the larger ornamentation, also separate it from T. impressa. 



I 



