BELEMNITES OF THE LIAS. 



73 



Transverse sections elliptical, with flattened sides ; across the anterior part of the 

 alveolar region, the unequal thickness of the ventral and dorsal parts of the guard is often 

 conspicuous ; and, what is remarkable, in specimens which have the one short apicial 

 furrow, it is on this side that the thickening is greatest. Such a groove usually indicates 

 the ventral aspect, and it is so figured by Quenstedt (' Cephal.,' pi. xxvi, fig. 1 d). Voltz, 

 who observed this unequal thickening, and paid much attention to it, states that, while 

 from the apex toward the alveolar cavity the deposited layers thicken most of all the 

 dorsal parts, and least of all the ventral, it is quite otherwise in the alveolar region, where 

 the augmentation takes place on the ventral part (Voltz, ' Obs. sur les Bel.,' p. 47, and 

 pi. ii, fig. 5 f"). He also remarks that the "summit" is oblique, and nearer to the ventral 

 than to the dorsal aspect. I find, indeed, that it is on this side, toward wdiich the apex 

 is inclined, that the short apicial groove occurs, when it does occur, contrary to what is 

 usual ; and it is on this side that the circum-alveolar space is thickened. Before, how- 

 ever, deciding that Voltz is altogether right in this observation, I wish to obtain 

 specimens from the Upper Lias of Gloucestershire, which may be properly cut for 

 examination. 



In a longitudinal section, the axis is seen to be very oblique, a little arched toward 

 and near to the ventral side, and marked by a pale tint in the laminae about it — probably 

 they were less calcareous. 



Proportions in the adult. Taking the dorso-ventral diameter at the alveolar apex at 

 100, the ventral part is 40, the dorsal part 60, the cross diameter 80, the axis 180 

 to 220. 



Phragmocone. Oval in section, slightly arched toward the ventral region, and 

 terminated by a spherule ; the angle of inclination of the sides 25° according to Voltz, 

 20° to 22° according to D'Orbigny. Voltz describes the summit of the dorsal " ogives " 

 as nearly rectangular. The diameters are 100 to 87. 



Varieties, a. Short, with obtuse ovoid summit inclined toward the grooved side of 

 the apex (PI. XV, fig. 37). 

 i3. Longer, with obtuse summit (PI. XV, fig. 39). 



Locality, var. a, short, obtuse. Frocester HiU (Sands), in the Cabinet of Bristol 

 Institution, two specimens marked b 1, No. 28. Also two specimens from the same 

 locality, in the Cabinet of Mr. Moore, one of them longer in proportion. 



Belemnites regtjlaris, n. s. PI. XV, fig. 38. 



Guard. Straight, cylindroidal, tapering in a curve to the end, compressed, with an 

 oval or elliptical section, and a three-grooved, somewhat striated, umbilicated termination ; 



