94 



DIBRANCHIATE CEPHALOPODS. 



Section I. Acoeli. 



"Without dorsal or ventral grooves. 

 Subsection 1. Acuarii. 



Without lateral furrows, but often channelled at the extreme 

 point. {Ex. B. acuarius. Lias.) 

 Suh-section 2. Clavati. 



With lateral furrows. (Ex. B. clavatus. Lias.) 

 Section II. Gastrocoeli. 



Ventral groove distinct. 

 Sub-section 1. Canaliculati. 



No lateral furrows. (Ex. B. canaliculatus. Inf. Oolite.) 

 Slib-section 2. Hastati. 



Lateral furrows distinct. (Ex. B. hastatus. Oolite.) 

 Section III. Notocoeli. 



With a dorsal groove, and furrowed on each side. {Ex. B. 

 dilatatus. Neocomian.) 



The species of the genus Bdemnitcs range from the top of 

 the Trias, where the earliest forms appear, to the Upper 

 Greensand, in which the genus, with one or two exceptions, 

 finally disappears. The species are most numerous in the 

 Jurassic rocks, and often occur in the greatest abundance 

 in particular beds or particular localities. It would seem 

 not improbal^le that the genus Bdojptera, 

 ,^^ before noticed, should be referred to the 



Bdemnitidce, and the genus Bclcmnosepia (or 

 Gcotcuthis), formerly referred to the Tcuthidce, 

 appears to be almost certainly referable here. 

 h. Belemnitella. — In this genus (fig. 479) 

 the skeleton is very similar in its general 

 arrangement to that of Belemnites ; but there 

 is a straight fissure in the guard, at its upper 

 end, on the ventral side of the wall of the 

 alveolus, and the surface is marked wdth dis- 

 tinct vascular impressions. The species of 

 this genus are exclusively Cretaceous, and are 

 only found in the upper portion of this for- 

 mation, ranging from the Upi^er Greensand to 

 the Chalk, 

 c. Bclemnotcuthis. — "Shell consisting of a jj/iro^macont', like 

 that of the Belemnite ; a horny dorsal peii with obscure 

 lateral bands ; and a thin fibrous guard, with two diverging 



Fig. 479.— Bc?«m- 

 nitello, mucronata. 

 Chalk. 



