J. S. GARDNER ON THE CRETACEOUS DENTALIID^l. 59 



D'Orbigny gives an excellent figure of the species in the ' Paleon- 

 tologie Franchise ;' but Alth's figure in Haidinger's Abhandlungen is 

 more slender, has stronger ribs, and is perhaps distinct. 



This species is everywhere abundant in the Gault. Casts are 

 common at Cambridge. 



D'Orbigny, in the ' Paleontologie Francaise,' unites B. decussatum 

 and B. ellipticum ; and I have now satisfied myself that B. ellip- 

 ticum is really an old and thickened form of B. decussatum. It 

 is slightly elliptical externally, but retains the cylindrical inner 

 tube, very thick near the apex, margin thin. The sculpture 

 has nearly entirely disappeared, there being more or less ob- 

 scure traces of ribs only ; and the shell is marked by uneven lines 

 of growth, giving it a weathered and generally coarse and clumsy 

 appearance. The cast, owing to the thickness of the shell, is slightly 

 funnel-shaped or bell-mouthed. 



This form is described and figured by Sowerby in the ' Mineral 

 Conchology,' vol. i. p. 161, tab. 70. f. 6, 7. He had seen but short 

 pieces of the thicker end of the shell, and named it from the ellip- 

 tical aperture. A similar fragment is figured in MantelTs ' Geology 

 of the South Downs ;' and the name also occurs in Fitton's list of 

 Elackdown fossils in the Strata below the Chalk (Trans. Geol. Soc. 

 2nd ser. vol. iv. p. 362). It has been very generally assumed, and 

 Stoliczka states, in the ' Palseontologia Indica,' that " B. ellipticum 

 is known to be a cast of B. decussatum ;" but the shell was not 

 described from a cast. It has no distinctive specific characters ; and 

 the occasional remnants of ribbing generally visible prove it to be a 

 very old form of B. decussatum much thickened, with the ribs 

 obliterated by age. 



It is common in the Gault, usually in the lower part ; and casts 

 are abundant at Cambridge. 



Dentalium medium, Sby. BlackdownBeds. (Plate III. figs. 13, 14.) 



Shell cylindrical, slightly curved, slender, thin : apex small, 

 entire : sculpture, more or less regular longitudinal ribs, of which 

 18 or more of the most prominent are persistent from the apex, the 

 rest increasing, as in B. decussatum, to about 40, most pronounced 

 on the convex surface • on some specimens these ribs are tolerably 

 regular, on others very irregularly spaced, crossed by indistinct 

 lines of growth and varices. None of the specimens met with 

 appear to be perfect, and therefore the dimensions are unknown ; 

 the longest specimen in any collection I have seen is about 1*1875 in. 

 B. -2083 to -25 in. 



This species is ribbed in the same manner as B. decussatum ; but, 

 owing to its mineralized condition perhaps, the ornament is far less 

 sharp. There are no transverse striae, but only lines of growth. 

 The aperture, in the specimens known to me, is invariably imper- 

 fect, probably through carelessness in extracting them from the 

 matrix. Should any of these be nearly perfect and full-grown, 

 which does not appear to me to be the case, they would be distin- 

 guished from B. decussatum by being ribbed all over. 



B. medium was figured in the ' Mineral Conchology' (vol. i. p. 181, 



