60 



BRITISH BELEMNITES. 



Belemnites subtenuis, Simpson. PI. X, fig. 27. 



Reference. Belemnites subtenuis, Simpson, Lias Fossils, No. 12 (excl. a and /3), 

 p. 26, 1855. 



Guard. Very elongate, compressed, tapering uniformly to the apex, which is marked 

 by three faint, though rather long furrows, and many fine striae. 



Sections show the outline to be elliptical, the axis very excentric and straight. 



Dimensions. Largest specimen in my collection 4 inches long, the greatest diameter 

 (uncrushed) less than \ an inch. 



Proportions. The diameter, v d, at the apex of the phragmocone being taken at 100, 

 the ventral radius is 36—38, the dorsal 62 — 64, the axis 700 to 1000, the diameter 

 from side to side 76. Nearer the apex the diameters are less unequal. 



Phragmocone. Visible, but not distinctly observable in the expanding anterior region. 

 The flanges of the septa appear very short; the cross section is elliptical, but not 

 so much as the section of the guard, which is thinner on the sides than on the dorsal or 

 ventral face. 



Locality. Above the Jet rock in Upper Lias at Whitby, abundant {Simpson, Phillips). 

 Robin Hood's Bay, in Upper Lias {Cullen). 



Observations. Simpson's description of B. subtenuis is in the following words: — 

 " Slender, regularly tapering, with three long shallow grooves toward the finely striated 

 apex." He allows two varieties from the type, viz. : — 



Var. a. Grooves obsolete, apex not striated. 



h. Thicker in proportion, apex not striated. 



These may probably be better referred to B. striolatus (see p. 59). The group thus 

 becomes definite, and may be compared with Simpson's larger but similar straight 

 tripartite forms, viz., B. trisulcosus and B. incisus, which, it appears possible, may 

 be full-grown individuals of the same species (see also p. 62). 



