60 BRITISH CRETACEOUS BRACHIOPODA. 



Terebratula sella, TfOrbigny. Pal. Franc., Terrains Cretaces, vol. iv, p. 91, pi. 510, 

 figs. 6 — 12, and Prodrome, vol. ii, p. 120, 1850. 

 — — Catalogue of the Terebratulse in the British Museum, p. 28, 1853. 



Diagnosis. Shell of a sub-quadrangular or somewhat pentagonal shape, rather longer 

 than wide. Valves almost equally convex, slightly flattened ; beak short, not much in- 

 curved, and obliquely truncated by a foramen of moderate dimensions, partly margined by 

 a wide and short deltidium in two pieces. Dorsal or socket valve more or less prominently 

 biplicated ; the front is considerably elevated, narrow, regularly arched or bisinuated ; 

 lateral margins flexuous ; surface smooth, with a few concentric lines of growth. Loop 

 short, not exceeding a third of the length of the socket valve. Dimensions variable : 

 length 15, width 14, depth 8 lines. 

 » 15, „ i'Z, ,, J ,, 



06s. Sowerby observes that " when young, this shell is rather trigonal, in consequence 

 of the length of the sides and roundness of the front ; as it grows older, it becomes squarer, 

 the front being more produced as well as more elevated ; the beak is very slightly curved ; 

 the length and breadth are very nearly equal ; the edges always sharp." But we may 

 add, that the separation of the plaits, as well as the depth of the sinus between 

 them, is very variable according to specimens, and is even at times almost entirely 

 filled up. 



Several authors, among whom we may quote Baron V. Buch and Professor Bronn, have 

 considered the species under description to be only a variety or synonym of Ter. bipli- 

 cata, but M. D'Orbigny justly remarks, that although individuals of Ter. sella are at times 

 found bearing some of the external aspect of T. biplicata, they are commonly well dis- 

 tinguished by their beak, foramen, deltidium, and position of the plaits, the last being 

 much more elevated and closer in T. sella, producing a biplicated mesial fold, and a deep 

 sinus in the ventral or dental valve. Tt cannot be confounded with Ter. prcrtonga, which 

 is a much more elongated or scuttle-shaped shell. 



In England, Ter. sella abounds in the Lower Green Sand of Atherfield (Isle of Wight), 

 at. Reigate, Pluckley, Ashford, Sandgate, and Hythe ; it is likewise (but rarely) found 

 in the Gault of Maidstone, w r hence an undeniable example was obtained by Mr. 

 Bowerbank ; and it is probable that some individuals continued to exist in the lower beds 

 of the Upper Green Sand, an opinion arrived at from the inspection of a few uncertain 

 examples found at Warminster and Farringdon. 



On the Continent, M. D'Orbigny mentions having found it in the Terrain Ncocomien 

 Superieur and Aptien of Combles, Gargas, Renaud-du-Mont, aux Salles, Castellane, 

 Marolles, &c. 



Plate VII, fig. 4. A remarkably adult and characteristic example, from the Lower 

 Green Sand of the Isle of Wight ; British Museum. 

 ,, fig. 5. From a bed of the same age, at Pluckley; in the Collection of Mr. Harris. 



