220 SUPPLEMENT TO THE BRITISH 



228. Rhynchonella Stephensi, Dav. Sup., PL XXVII, fig. 20. 



Shell small, transversely oval, broadest posteriorly, tapering anteriorly, valves almost 

 equally deep. Dorsal valve smooth, and uniformly convex to about two thirds of its 

 length, when a wide biplicated fold is produced along with two or three wide but short 

 ribs on each of the lateral submarginal portions of the valve. In the dorsal valve a wide 

 and deep sinus extends from the middle of the valve to the front, with one central rib. 

 Two or three short ones also occupy the lateral portions of the valve. Beak small, 

 incurved ; foramen minute. 



Length 4, width 5, depth 2\ lines. 



Obs. — Mr. Darell Stephens found two examples of this small, but well-marked, 

 species in the Inferior Oolite at Crewkerne Station, near Sherborne. It bears some 

 resemblance to one or two RhynchonellcB described by Messrs. Chapuis and Dewalque, 

 from the Lias of Luxemburg, but differs from them in size and other respects, also 

 in stratigraphical position ; and, as I could find no named species from the Inferior 

 Oolite with which to identify it, I have thought it preferable to give it a separate 

 designation. 



229. Rhynchonella calcicosta, Quenstedt. Dav., Sup., PI. XXVIII, figs. 24 — 33. 



Terebratula calcicosta, Quenstedt. Handbuch Petrefact., p. 451, pi. xxxvi, figs. 



6—9, 1851. 

 — — Quenstedt. Die Jura, p. 138, pi. xvii, figs. 16, 17, 1858. 



Shell more or less subtrigonal or subpentagonal, as broad as long, or a little wider 

 than long. Dorsal valve divided into three lobes, the middle one forming a moderately 

 raised, flattened, mesial fold. Sinus in ventral valve wide ; beak much incurved ; foramen 

 small, not always visible, margined by small deltidial plates. Surface marked with 

 strong ribs, from fourteen to twenty in number, which radiate direct from the extremity 

 of the beak to the margin ; two to six form the mesial fold and sinus. When young 

 the shell is almost circular, and uniformly convex, without any fold or sinus, and with 

 almost equal-sized ribs. Occasionally a shorter rib is interpolated between two larger 

 ones ; and the shell is, as an exception, sometimes slightly longer than wide. Proportions 

 variable — 



Length 6, width 7, depth 4^ lines. 



Obs. — Two closely allied species, or varieties, have been described and figured by 

 Quenstedt, under the designation of calcicosta and plicatissima. Some palaeontologists 

 consider them to be distinct species, others varieties, and some merge them into one. 



