74 MANUAL OP THE MOLLUSCA. 



divided into air-chambers, connected by a small tube (siphuncle), like the shel 

 of the pearly nautilus. It is exceedingly delicate, and usually owes its preser 

 tioii to the infiltration of calc. spar ; specimens frequently occur in the lias 

 with the meniscus-shaped casts of the air-chambers loose, like a pile of watch, 

 arlasses. It is usually eccentric, its apex being nearest to the ventral side o: 

 the guard. The guard is very variable in its proportions, being sometimes 

 only half an inch longer than the phragmocone, at others one or two feet ii 

 length. These variations probably depend to some extent on age and sex 

 M. D'Orbigny believes that the shells of the males are always (comparatively) 

 long and slender ; those of the females are at first short, but afterwards grow- 

 ing only at the points, they become as long in proportion as the others. Thjt 

 guard always exhibits (internally) concentric lines of growth; in B. irregularu. 

 its apex is hollow. The belemnites have been divided into groups by the pre- 

 sence aud position of furrows in the surface of the guard. 



SECTION I. ACCELI (Bronn.) without dorsal or ventral grooves. 

 Sub-section 1. Acuarii, without lateral furrows, but often channelled at 

 the extreme point. 



Type., b. acuarius. 20 sp. Lias Neocomian. 

 Sub-section 2. Clavati, with lateral furrows. 

 Type, b. clavatus. 3 sp. Lias. 



SECTION II. GASTJIOCCELI (D'Orb.) Ventral groove distinct. 

 Sub-section 1. Canaliculati, no lateral furrows. 

 Type, b. canaliculatus. 5 sp. Inf. oolite Gt. oolite. 

 Sub-section 2. Hastati, lateral furrows distinct. 

 Type, b. hastatus. 19 sp. U. lias Gault. 



SECTION III. NOTOCCELI (D'Orb.) with a dorsal groove, and fir 

 each side. 



Type, b. dilatatus. 9 sp. Neocomian. 



The belemnites appear to have been gregarious, from the exceeding abuiul- 

 ance of their remains in many localities, as in some of the marlstone quarrie- 

 of the central counties, and the lias cliffs of Dorsetshire. It is also probabL 

 that they lived in a moderate depth of water, and preferred a muddy bottom 

 to rocks or coral-reefs, with which they would be apt to come in perilous col- 

 lision. Belemnites injured in the life-time of the animal have been frequently 

 noticed. 



BELEMNITELLA, D'Orb. 



Syn., actinocamax, Miller (founded on a mistake.) 



Type, B. mucronata, Sby. PL II., fig. 6. 



Distr., Europe; N. America. 5 sp. U. greensand and chalk. 



The guard of the belemnitella has a straight fissure on the ventral side < I 

 its alveolar border ; its surface exhibits distinct vascular impressions. The 



