INOCERAMUS. 285 



Fig. 41. — Jnoceramus labiatus var. latus, Sow. Zone of Holaster planus, Coombe Bissett. Dr. Blackmore's 



Collection. Right valve. Natural size. 



Inoceramus inconstaxs, sp. nov. Plate LI, figs. 1 — 4. Text-figs. 39, 42 — 49. 



1822. Inoceramus Beongniarti, var., G. Mantell. Foss. S. Downs, p. 215, pi. 



xxviii, fig. 3. 

 sp. Mantell. Ibid., p. 217, pi. xxvii, fig. 9. 

 1850. — La.marckii, J. de C. Sowerby in F. Dixon. Geol. Sussex, p. 



355 (p. 385, ed. 2), 

 pi. xxviii, fig. 29. 

 1904. — Beongniaetii, C. Airaghi. Boll. Soc. geol. Italiana, vol. xxiii, 



p. 192, pi. iv, figs. 3—5. 



Description. — Shell extremely inequilateral, moderately or slightly inequivalve; 

 the early part of the shell slightly convex or nearly flat, the later part much more 

 convex and sometimes growing nearly perpendicularly to or forming a large angle 

 with the early part, so that in old individuals the shell becomes more inflated ; in 

 other cases the convexity increases more gradually from the earlier to the later 

 stages of growth and a nearly globose shell results. Height of the shell often 

 rather greater than the length. Hinge-line long, forming rather more than a 

 right angle with the anterior margin, which is usually more or less nearly straight. 

 Ventral and posterior margins rounded. Umbones terminal, small, pointed, only 

 slightly curved. Anterior marginal part of the valve usually flattened, but not 

 definitely limited from the sides of the shell. Posterior ear developed but not 

 distinctly limited. 



Concentric folds are usually well developed, but become indistinct on the 

 anterior flattened area, on the posterior ear, and on the later part of the valve 

 of old individuals. The concentric folds have a very unsymmetrical curvature. 



Remarks. — The forms included in this species vary considerably, owing mainly 

 to the length of duration of the flattened stage, which in some individuals forms a 



38 



