64 



BRITISH BELEMNITES. 



Proportions. Taking the diameter from back to front at the alveolar apex at 100, 

 the cross diameter is about 90, but this varies in different specimens ; the excentricity of 

 the axis is variable, in some young specimens the ventral is only 28 to the dorsal 72 ; 

 in another specimen the ventral is 40, the dorsal 60; in another, ventral 48, dorsal 52; 

 the axis is from 300 in middle-aged to 500 or 600 in the young. In Voltz's figure 

 the axis is 340. 



Phragmocone. Very slightly arched ; its section oval, with proportions of 100 to 93 ; 

 septa close, rather unequally arranged, but in the middle part, on the average, distant 

 one seventh of the diameter, apparently formed of a single plate with a short flange. In a 

 specimen from Whitby an extraordinary number of the septa are squeezed together in the 

 hinder part of the last chamber, by pressure from without. If these were supposed to 

 be the last septa, and to be replaced, the axis of the phragmocone would equal half the 

 axis of the guard. The angle appears to be 24^30' (Voltz gives 25°). 



Locality. In the Upper Lias shale of Whitby and Saltwick {Phillips). 



Belemnites Ilminstrensis, n. s. PI. XII, fig. 30. 



Guard. Elongate, straight, conoidal, more or less compressed, very gradually 

 tapering over the whole of the guard to an acute point (24°), often cross-banded with 

 light and dark shades ; axis subcentral when young, more excentric when old ; one 

 ventral, two dorso- lateral furrows, all reaching the apex, the ventral furrow usually 

 longest of the three. In young specimens the guard is depressed about the end, and 

 shows a broad ventral furrow there, with slight and short traces of the dorso-laterals. The 

 sides are often slightly canahculate for the whole length in young specimens. 



Dorsal aspect rather broader than the ventral. 



Sections show great excentricity of the axis in full-sized specimens, and in adults an 

 oval section. In young specimens which are depressed, Avith flattened sides, the section 

 of the guard is subquadrate. 



Dimension?,. Greatest length of axis of guard observed, 5 inches ; greatest diameter 

 of the most expanded part, 1 inch. 



Proportions. Taking the dorso-ventral diameter at the alveolar apex at 100, the 

 ventral radius is 33 -}-, the dorsal 66 -|-, the cross diameter 85 ; the axis in a very long 

 compressed variety (axis 4-5 inches long) 800, in a shorter variety 600, in one still 

 shorter 400, in the shortest 350. These are adults. In a young specimen the axis 

 (1 inch long) is 350, in another (axis 0*6 inch long) 300, in the shortest of all the 

 specimens (less than \ inch long) the proportion is nearly the same. 



