BIVALVIA. 61 
umbones are larger ; the concentric rugz will also at once distinguish it, as the young shell 
of 8. Madridi is smooth. 
Geological Positions and Localities. 'The Great Oolite of Hampton cliffs ; the Corn- 
brash of Laycock. In the collection of W. Walton, Esq. 
Opis Lecxrnsy1, Wright. ab. XXXVII, figs. 9, 9 a. 
Oris LeckenByI, Wright, in Proc. Geol. Soc., vol. xvi, part 1, 1860. 
Testa crassa, trigona, obliqua, fornicata, inequilatera, cordiformt, transverse regulariter 
costata, postice acute carinata, umbonibus magnis elevatis, anticis, involutis, latere antico 
brevissimo, postico subrecto oblique declivi; lunula magna profunda, marginibus obtusis, 
striatis ; costis transversis, regularibus, angustis, subacutis ; valvis striis longitudinalibus et 
decussantibus subtillissimis instructis. 
Shell thick, trigonal, oblique, very convex, and ineequilateral, cordiform, with transverse, 
regular cost ; a large, flattened, posterior area is separated from the other portion of the 
shell by an elevated, acute carina, anterior and parallel to which is a slight depression ; the 
umbones are large, elevated, much inclined forwards, and involute; the anterior side is 
very short, having a large and deep Junule, whose margin is rounded and striated elite 
cost upon the sides of the valves are regular, narrow, subacute, and not much elevated ; 
the wide, posterior area has large, oblique striations; the costated portion is covered with 
extremely fine perpendicular and decussating striations, which are only distinguishable 
under a magnifier. 
Height, 15 lines; length, 15 lines ; diameter through both the valves, 13 lines. 
A large and elegant species, distinguished from Opis dunulatus, Sow., by the more con- 
vex figure, the rounded margins of the lunule, and by the more acute and more densely 
arranged costa; the posterior carina and bordering sulcation are also very prominent 
features ; the costze under a magnifier exhibit a beautifully decussated surface. 
Geological Position and Locality. The Cornbrash of Scarborough ; a single specimen 
in the collection of Mr. Leckenby. 
Opis putcHELLA, D’Orb. Part II, Tab. VI, fig. 3, p. 80. 
Opis PULCHELLA, D’Orbigny. Prodrome, i, p. 307. 
— LUNULATUS, var. Great Ool. Mon., Pal. Soc., part 2, pl. 6, fig. 6, p. 80. 
Espeéce voisine de lO. lunulata, mais bien plus courte et moins oblique, presque carrée 
ornée de cotes concentriques.” (D’Orbigny.) 
The experience derived from a multitude of examples leaves no room to doubt that the 
