TURRITELLA TRICARINATA. t39 



only because they differ in form and ornament, as will be seen in the figures now 

 given (PI. XLIV, figs. 1 to 9), but also in their range both of time and space, the 

 one T. trvplicata being a southern form widely diffused in Miocene strata but now 

 extinct, the other T. communis a shell with a somewhat more northerly distribution, 

 indicating a comparatively late appearance in the Crag basin and becoming subse- 

 quently an exceedingly abundant Pleistocene species which is still living. Brocchi 

 and Prof. Sacco describe these two forms as distinct in the Italian Pliocene, either 

 as specific or varietal, while Seguenza gives both of them from different horizons 

 of the Sicilian deposits. 



I have figured a fossil specimen of T. tricarinata from Sicily with others from 

 the English Crag to show the identity of the latter with the Mediterranean 

 fossils. 1 In T. tricarinata the spiral sculpture is more delicately and distinctly 

 chiselled than in T. communis and is decidedly tricarinate. 



Var. bicincta (S. V. Wood). Plate XLIV, fig. 10. 



1842-48. Turritella bicincta, S. V. Wood, Ann. Mag. Nat. Hist, [1], vol. ix, p. 534, 1842 ; T. incras- 

 sata, var. bicincta, Mou. Crag Moll., pt. i, p. 75, pi. ix, fig. 7 d, 1848. 



1879. Turritella incrassata, var. bicincta, Cogels and Van den Broeck, Ann. Soc. Boy. Malac. Belg., 

 vol. xiv, p. 71. 



1912. Turritella tricarinata, var. bicingulata, Cerulli-Irelli, Balaeont, Ital., vol. xviii, p. 159, pi. xxiv, 

 fig. 26. 



Varietal Characters. — Corresponds generally with T. tricarinata, but it has two 

 well-marked prominent and equal spiral ridges in the centre of each whorl instead 

 of three, as well as some excessively fine lines, hardly visible without the aid of a 

 lens. On the lowest whorls the base is spirally thickened immediately above the 

 suture, so as to present the appearance of a third ridge, which angulates the base 

 of the shell. 



Dimensions. — L. 25 mm. B. 8 mm. 



Distribution. — Not recorded living. 



Fossil : Coralline Crag. Waltonian : Walton-on-Naze, Little 

 Oakley (abundant). Newbournian : Felixstow, probably elsewhere. Butleyan : 

 Butley. Zone a Isocardia cor : Antwerp, Belgium. 



Remarks. — This shell is fairly common in the earliest horizons of the Crag, 

 occurring abundantly with the type form of T. tricarinata, both in the Coralline 

 Crag and in the Waltonian of Walton and Oakley. It was regarded at first by 

 Wood as a distinct species, but afterwards both by him and by M. Van den Broeck 

 as a variety of T. incrassata. Its affinities, however, are with T. tricarinata, 

 resembling that species in size, form, and general appearance. A similar shell 



1 Dr. Lorie's figure of T. terebra {up. cit.) represents a typical specimen of T. tricarinata. 



57 



