70 



BRITISH BELEMNITES. 



apicial region, grooved and flattened, 1 inch ; post-alveolar region, uniformly tapering, 

 7i inches ; alveolar region of visible guard, 2 inches ; projection of phragmocone beyond it, 

 li inch. The phragmocone is extended to a total length of nearly 3| inches, all cham- 

 bered, with a terminal breadth, completely flattened, of ll inch. This gives an apparent 

 angle to the phragmocone of 27°; but if reduced to a cone, about 18°. The diameter of 

 the guard at the alveolar apex is about 0-375 inch ; the length of the axis, twenty times 

 as great. Nearly all the surface is striated, the striae being everywhere undulated and 

 interrupted, and largest and most conspicuous on the alveolar region. 



Another fine specimen also presented by Dr. Plumptre to the Oxford Collection 

 is Hi inches long; of this the posterior 7fths inches are compressed flat; 3 inches 

 remain quite unaffected by pressure, and beyond this is 1 inch of crushed alveolar 

 cavity. Diameter at the alveolar apex *oths of an inch. Three parallel grooves, which 

 seem to be too regular to be the effect of mere crushing, appear on a great part of the 

 flattened post-alveolar region ; toward the point only one is traceable, and it is accompanied 

 by several striae. The grooves referred to do not extend over the solid post-alveolar and 

 alveolar tracts ; on which, however, facettes can be traced which seem to correspond with 

 the grooves. From this it seems, on the whole, probable that the grooves are the effect 

 of uniform pressure on the unequally resisting facetted surface of the hollow part of 

 the guard. 



The shells which commonly accompany this Belemnite are chiefly Posidonia 

 {Inoceramus dubius of the ' Min. Conch.') ; and the shale may well be called, as in 

 Wiirtemburg, where B. acuarius occurs in it, the Posidonian shale. 



Belemnites acuarius, Schlotheim. (Diagram No. 22.) 



Reference. Schlotheim, 'Petref.,' p. 46, No. 2, 1820; D'Orb., 'Terrains Jurassi- 

 ques,' pi. vii, referred to in his text, p. 76, as pi. v; Quenst., ' Jm'a,' 

 p. 258, pi. xxxvi, fig. 9, 1857. 



Guard. Elongate, compressed, striated on the sides in the retral part, and 

 flattened, by compression, at the end. 



Transverse section oval, with somewhat flattened sides ; the ventral surface broadest, 

 axis nearer to the ventral side. 



Taking the diameter from back to front at 100 ; the dorsal radius is 66, the ventral 

 34, the cross diameter 73. 



Phragmocone not seen in English specimens. 



Locality. Cheltenham, in the Belemnite-bed of the Lower Lias {Bnckman). 



