80 



BRITISH BELEMNITES. 



the young of the Voltzian shells, being quite devoid of ventral sulcus, but they are com- 

 monly acute, and not striated ; others, which are acute and striated, with a ventral sulcus, 

 may be thought to be the young of B. ventralis. 



Belemnites ventralis, n. s. PI. XVII, figs. 44, 45. 



Guard. Conoidal, compressed, smooth, with a central blunt or eroded termination 

 (when old). Two dorso-lateral furrows and one medio-ventral groove proceed along 

 nearly half the space between the end of the guard and the end of the alveolus. The ter- 

 mination is striated, and the striae extend to about half the length of the grooves. 



Transverse sections at the apex of the alveolus show the axis excentric and the 

 contour regularly oval ; the ventral side rather narrower than the dorsal in the medial 

 region, but wider in the part near the apex. Further backward the section shows the 

 effect of the ventral and lateral grooves. The axis is nearly straight. The greatest 

 observed length, including about one third of the phragmocone, is 6 inches ; the greatest 

 diameter \\ inch. 



Proportions. The diameter {v d) at the apex of the alveolus being taken at 100, 

 the ventral radius is 41, the dorsal 59, the axis 300 to 850, the cross diameter 90. 



Phragmocone. Slightly arched, ending in a spherule, with a slightly oval section 

 (100 to 95 or 90) ; septa obliquely descending to the siphuncle, and waved on the edge ; 

 the depth of the chambers equal to one seventh of the diameter. The sides are inclined 

 to each other 21°. The conothecal striation is very distinct. 



Localitij. Upper Lias of Whitby, Saltwick, and Robin Hood's Bay {Phillips). 

 Frocester Hill Sands [Moore). Not observed at Lyme Regis {Phillips). 



Belemnites inornatus, n. s. PI. XVIII, fig. 46. 



Beferencc. Belemnites compressns, Blainville, 'Belemn.,' p. 84, pi. ii, fig. 9, 1825. 



Guard. Conoidal, compressed, smooth, Avith a blunt or eroded termination (when 

 old). Two dorso-lateral furrows proceed along one third of the apicial space. No ventral 

 furrow, no striae about the termination. 



Transverse sections at the apex of the alveolus give an excentric axis and a regularly 

 oval contour, the ventral portion being rather narrower than the dorsal. Toward the apex 

 the lateral grooves indent the outline. 



Greatest length observed, 4'65 inches, of which the axis is 2'65. 



