BIVALVIA. 61 



umbones are larger ; the concentric rngse will also at once distinguish it, as the young shell 

 of S. 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 LECKENBYI, Wright. Tab. XXXVII, figs. 9, 9 a. 



OFIS LECKENBYI, Wright, in Proc. Geol. Soc., vol. xvi,.part 1, 1860. 



Testa crassa, trigona, obliqua, fornicata, inesquilafera, cordiformi, transverse regulariter 

 costafa, postice acute carinata, umbonibus magnis elevatis, anticis, involutes, la/ere antico 

 brevissimo, postico subrecto oblique declim ; lunula magna profunda, marginibus obtusis, 

 striatis ; costis transvcrsis, regularibus, angustis, subacutis ; valvis striis longitudinalibus et 

 decussantibus subtillissiinis instructis. 



Shell thick, trigonal, oblique, very convex, and inequilateral, cordiform, with transverse, 

 regular costa? ; 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 lunule, whose margin is rounded and striated ; the 

 costse 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 Opts lunulatus, Sow., by the more con- 

 vex figure, the rounded margins of the lunule, and by the more acute and more densely 

 arranged costae ; the posterior carina and bordering sulcation are also very prominent 

 features ; the costse 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 PULCHELLA, V Orb. Part II, Tab. VI, fig. 3, p. SO. 



OPIS PULCHELLA, D'Orligny. Prodrome, i, p. 30". 



LUNULATUS, var. Great Ool. MOD., Pal. Soc., part 2, pi. 6, fig. 6, p. SO. 



Espece voisine de I'O. lunulata, maisbienplus court e et moins oblique, presque carree 

 ornee de cotes concentriques." (D'Orbigny.) 



The experience derived from a multitude of examples leaves no room to doubt that the 



