RHYNCHONELLA. 93 



05s. This elegant species was discovered by the Rev. A. W. Griesbach, in the 

 Cornbrash of Rushden, in Northamptonshire, and has likewise been found in the same 

 formation at Marquise, near Boulogne, by M. Bouchard. This form, although approaching 

 to B. obsoleta in some respects, seems to me not to possess all the characters belonging 

 to that species ; it is more elegant and somewhat winged shape, smaller, has more delicate 

 plaits, and its prominent mesial fold, as well as general aspect, appear to distinguish it 

 from the types of obsoleta, as found in the Great Oolite and Bradford clay ; if not specifically 

 distinct, it would be at least a well-marked variety. After submitting the question to 

 various competent persons whose opinions likewise differed, I have ventured to distinguish 

 it by a separate appellation. 



Plate XVIII, fig. 12. From a specimen from the Cornbrash of Rushden, in the 



collection of the British Museum. 

 „ fig. 1 3. From one in that of the Rev. A. W. Griesbach. 



88. Rhynchonella tetraedra, Sow. Sp. Plate XVIII, figs. 5 — 10. 



Terebratula tetraedra, Sow. M. C, vol. i, 1812, p. 191, tab. Ixxxiii, fig. 5. 



— — Parkinson. An Introduction to the Study of Organic 



Remains, p. 227, 1822. 



— media, lb., p. 234. 



— tetraedra, Defrance. Die. d'Hist. Nat., vol. liii, p. 158, 1828. 



— — Young and Bird. Geological Survey of the Yorkshire 



Coast, pi. viii, fig. 15, 1828. 



— — V. Buck. 1834. Uber Ter.; and Mem. de la Soc. Geol. de 



France, 1838, vol. iii, p. 139, pi. xiv, fig. 8. 



— — Scklotkeim. 1832. Systematisches Vers der Petrefacten. 



— — Deslongckamps. Soc. Linn, de Normandie, 1837. 



— — Morris. Catalogue, 1843. 



— — Tennant. A Strat. List of British Fossils, p. 74, 1847. 



Terebratula tetraedra, Bronn. Index Pal., p. 1253, 1848, (but not all his 



synonyms.) 



Rhynchonella tetraedra, D'Orbigny. Prodrome, vol. i, p. 258, 1849. 



Diagnosis. Shell variable in shape, obtusely deltoid, valves convex and gibbous, wider 

 than long ; beak acute, more or less recurved, almost touching the umbo in some species ; 

 foramen small, generally entirely surrounded by the deltidium ; beak ridges well defined, 

 leaving between them and the hinge margin a small concave false area, which indents more 

 or less the umbo of smaller valve, which is likewise laterally depressed, as if pinched in. 

 The greatest depth is, in some specimens, towards the middle, in others near the front. 

 Surface ornamented by a variable number of sharp plaits, from twenty-two to thirty on each 

 valve ; varieties with four, five, six, seven, eight, and nine plaits, forming a central more or 

 less elevated mesial fold, corresponding to three, four, five, six, seven plaits, in a deep 

 sinus in larger valve : on either side of the mesial fold are seen one or two plaits, which 



