RHYNCHONELLA. 87 



in fig. 8, and is specially remarkable from the convexity of the imperforated valve, which in 

 this case is by far the largest of the two. On some internal casts I have observed the 

 almost entire vascular system, the principal arteries becoming more and more subdivided 

 as they approach the margin of the shell or mantle. The beak is also, in some few instances, 

 and more commonly so in British specimens, much recurved, leaving little space for the 

 passage of the pedicle fibres ; but this is not the case with the generality of specimens 

 where the foramen is completely exposed, and more or less separated from the umbo by the 

 deltidial plates. Fig. 8 is drawn from a specimen in the collection of Dr. Wright. 



81. Rhynchonella inconstans, Sow. Sp. Plate XVIII, figs. 1, 2, 3, 4. 



Terebkatula inconstans, Sow. M. C, vol. iii, p. 137, tab. 277, figs. 3, 4, 1821. 



— — V. Buck, 1834. Uber Terebrateln; and 1838, Mem. de la 



Soc. Geol. de France, vol. iii, p. 146, pi. xiv, fig. 16. 



— — Beshayes, 1836. Nouv. ed. de Lamarck, p. 355. 

 — Pusch? 1837. Polens Pal., pi. iii, fig. 3. 



— — Beslong champs, 1837. Soc. Linn, de Normandie. 



— difformis, Zieten, 1832. Die Vers. Wurt., tab. xlii, fig. 2. (Non 



T. difformis,) Lamarck. 



— inconstans, Morris. Catalogue, 1843. 



— — Tennant. A Strat. List of British Fossils, p. 73, 1847. 

 Rhynchonella inconstans, B'Orhiyny, 1849. Prodrome, vol. i, p. 375. 



Diagnosis. Shell almost circular, more or less globose, unsymmetrical, rather wider 

 than long; beak acute and recurved, under which is seen a small foramen entirely 

 surrounded by the deltidium, and more or less separated from the umbo. Lateral ridges 

 well defined, leaving between them and the hinge margin a flat or concave false area, which 

 slightly indents the smaller valve. Valves convex, generally gibbous, without mesial fold or 

 sinus, one half at a little distance from the beak and umbo, becoming more elevated than 

 the other, which is turned down as if twisted, separating the shell into two parts ; in some 

 specimens the elevation is at the right, in others at the left side of the shell. Surface 

 ornamented by a variable number of simple plaits, from thirty to forty on each valve. 

 Length 19, width 22, depth 18 lines. 



Obs. B. inconstans seems to have been first figured by W. Smith, in or after 181 6, 1 

 from the Kimmeridge Clay. It was shortly afterwards named, figured, and described, 

 by Mr. Sowerby, under the above-mentioned denomination, that author observing, 

 "that one half of the edge is turned up and the other down, but indifferently to the 

 right or left." Professor Phillips 2 and Mr. Morris 3 mention it as found in the Speeton 

 Clay of Yorkshire, but I believe this to be a mistake ; the Speeton Clay being considered 



1 Strata identified by organised fossils, pi. x, fig. 6. 



2 Illustrations of the Geology of Yorkshire, 1831. 



3 Catalogue, 1843. 



N 



