SUPPLEMENT. 17 



the short posteal varices. Compared with the general aspect of the Undulata, the narrow 

 lengthened figure and promhience of the nodose varices upon the middle and posteal 

 portions of the valve constitute features which will not readily be mistaken for other of 

 "the Lower- Jurassic species, whether British or foreign. 



Dimensions. — Length measured upon the marginal carina 28 lines ; measurement at 

 right angles to the carina 20 lines; convexity of a single valve about 6^ lines. 



l^osition mid Locality. — The specimen contributed by Mr. E. A. Walford was obtained 

 from the Inferior Oolite of Oxfordshire, in the stratum above the Clypeus-grit, near 

 Stow-on-the-Wold. 



Tbigonia WiNDOESi, Li/cett. Sp. nov. Trigon. Supplement, Plate I, figs. 7, 8, 9, 10 



Shell in the very young condition much depressed, but acquiring a moderate con- 

 vexity with advance of growth, ovately trigonal ; umbones pointed, but little produced, 

 antero-mesial, and slightly recurved ; area large, flattened, divided by a median furrow 

 and bounded by carinae ; of these the marginal carina is moderately elevated and plain, 

 the inner carina is small, but is rendered serrated and rugose by the terminations of the 

 transverse costelte, which are plain, prominent, and become conspicuous as they approach 

 the inner border of the area. The sides of the valves have a numerous series of plain, 

 curved costse (about thirteen), the first-formed five or six are simply curved and united 

 posteriorly with the plain marginal carhia; those which succeed enlarge and curve 

 upwards at their posteal extremities, so that in the largest specimens the posteal portions 

 of the costa) become oblique or nearly perpendicular varices, which are partially disunited 

 from their anteal portions. There is much variability in the transverse costellae upon the 

 area, but usually they nearly disappear upon the area near to the siphonal border, which 

 is oblique, but shorter than the hinge-border. The escutcheon is narrow, depressed, 

 flattened, and inconspicuous. The entire plain marginal carina clearly separates the two 

 portions of the valves, and is of itself sufficient to distinguish this little species from the 

 Trigonia cuspidata of Sowerby, for which it has been mistaken ; the latter is a minute, 

 dwarfed, and very young form of a Great-Oolite species, probably T. Moretoni. 



This small species has occurred rather sparingly in the lower beds of the Inferior 

 Oolite at Hook Norton ; the largest specimen has a length of 9 lines, the height 

 being 7^ lines, and the convexity of a single valve 2^ lines. 



This species belongs to the Angulatse group, but with the general figure much shorter 

 or subovate, and more depressed ; the large area with its prominent costellae is also very 

 distinct ; the smooth costse are also quite destitute of tubercles. In the very young con- 

 dition it might be taken for one of the Costata. 



