74 MAXL'AL OP THE MOLLUSCA. 



dinded into air-chambers, connected by a small tube {siphunde), like the shell •, 

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

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

 wiXk the meniscus-shaped casts of the air-chambers loose, like a pile of \vatch- 

 glasses. It is usually eccentric, its apex being nearest to the ventral side of 

 tlie 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 in 

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

 ]\r. D'Orbigny believes that the shells of the males are always (comparatively) 

 long and sleuder ; 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. The 

 guard always exhibits (iuternaUy) concentric lines of growth; in B. irregulam 

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

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



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



Sub-section 1. Acuarii, vsdthout lateral fun-ows, but often channelled at 

 the extreme point. 



Tgpe., b. acuarius. 20 sp. Lias— Neocomian. 



Sub-section 2. Clavati, with lateral fm-rows, 



Tj/]je, b. clavatus. 3 sp. Lias. 

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



Sub-section 1. Canaliculati, no lateral furrows. 



Ti/pe, b. canaliculatus. 5 sp. Inf. oolite — Gt. oolite. 



Sub-section 2. Hastati, lateral fuiTows distinct. 



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

 SECTION III. NoTOCCELi (D'Orb.) with a dorsal groove, and furrowed on 

 each side. j 



Ti/pe, b. dilatatus. 9 sp. Neocomian. •', 



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

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

 of the central counties, and the lias chffs of Dorsetshire. It is also probable 

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

 to rocks or coral-reefs, \\-ith which they woidd be apt to come in perilous col- 

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

 noticed. 



Belemnitella, D'Orb. 



Syn., actinocaniax, MiUer (foimded on a mistake.) 



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



Disir., Em-ope ; N. America. 5 sp. U. greensand and chalk. 



The guard of the belemnitella has a straight fissm-e on the ventral side of 

 its alveolar border ; its sm-face exhibits distinct vascular impressions. The 



