TEREBRATULA. 39 



marginal line nearly straight, front more or less convex or concave, bounded, when old, by two 

 prominent angles alike in each valve, as if produced by the pinching of the edge ; beak pro- 

 duced, rounded, truncated by an entire foramen, separated from the umbo by a somewhat 

 long triangular deltidium in two pieces ; lateral ridges not continued to a great distance, 

 there existing a strongly-marked lateral flatness, the greatest width of the shell being 

 generally, though not always, at the front ; surface smooth, finely punctuated ; loop at- 

 tached only to the crura, and extending to near the margin of the shell. Dimensions and 

 form variable; average size, length 13, width 9, depth 8 lines. 



Obs. This is one of the first- described species in the Min. Con., and long known as a 

 characteristic oolitic fossil, and is stated by Sowerby, p. 217, to be "variable in its form, 

 sometimes almost globose, at others acutely triangular and rather depressed ; the two 

 angles of the front are continued a little along each valve, and look as if they were 

 produced by pinching the edges between the fingers ; the front between the angles is in 

 some shells concave, in others straight, or of different degrees of convexity." Von Buch's 

 description of this species would lead us to imagine that the greatest width of the shell 

 was always in front, since he states and underlines " Les aretes Cardinales descendent en 

 divergeant d'une maniere continue vers les cotes, et remplacent tout a fait les aretes 

 laterales, on autrement dit ne convergent pas, mais descendent verticalement de sorte que 

 la largeur du front est en meme temps la plus grande largeur de la coquille." Though 

 this is the general distinguishing character of digona, it often varies, as I have shown from 

 the original figures of the species, from Smith's Collection, deposited, in 1816, in the 

 collection of the British Museum, thus often approaching, in general form and convexity, 

 to certain specimens of Ter. obovata, but in all its varieties T. digona may be distinguished 

 by the lateral beak ridge not being recurved to join the hinge margin, and the lateral 

 angles at the front are never visible in obovata, the sides of the larger valve being in the 

 adult specimens usually flat and vertical, which is not the case in T. obovata. 



The figures I have given in Plate V will illustrate some of the varieties in form which are 

 presented by this species ; occasionally specimens are perfectly equilaterally triangular, others 

 are twice as long as they are wide, while some are as wide as they are long, and were well 

 figured and described by Mr. Smith, in 1816. It is a very common shell in the Bradford Clay 

 Forest Marble and Great Oolite ; it is found abundantly round Bath, Bradford, Cirencester, 

 &c, it occurs by millions at Ranville, in Normandy, and in other localities, and is also stated 

 to be found at Muggendorf, by Von Buch. Fig. 18 illustrates Mr. Smith's specimens, 

 now in the Collection of the British Museum. 



34. Terebratula obovata, Soio. PL V, figs. 14 — 17. 



Terebratula obovata, Sow. Min. Con., 1812, vol. i, p. 228, tab. 101, fig. 5. 

 — — Morris. Catalogue, 1843. 



— lyOrb. Prodrome, vol. i, p. 316, 1849. 



