CEPHALOPODA. 35 
as they rise over a slight linear elevation, which traverses the whole length of the 
alveolus, along the medial line of the ventral inner surface, evidencing the presence 
and position of the siphuncle. The opening, or anterior extremity of the conical 
cavity, is slightly elliptical, having the shorter axis in the direction from the ventral to 
the dorsal aspect. The margin of the outer sheath is thin and sharp, and its ventral 
paries is much thicker than the dorsal paries, and rises into an elevated mass, depressed 
on the surface. The outer sheath itself is composed of a series of concentric layers, 
and exhibits a fibrous texture, like the sheath of the Belemnite. The apex is pro- 
longed into a dense calcareous mass, strongly inflected towards the ventral aspect, 
and enlarged towards the posterior extremity, where it becomes attenuated, and is 
obliquely truncated. This mass is composed of longitudinal laminz, radiating from 
the apex of the cone, and so arranged, that the central lamine are ina plane extending 
from the ventral surface to the back, and the rest in planes gradually diverging more 
and more towards the back. The outer edges of the lamine are distinct and slightly 
elevated, giving a rough sulcated appearance to the surface. The cone and the 
calcareous mass into which it is prolonged expand laterally into two smooth semi- 
elliptical appendages, inclined obliquely towards the ventral aspect, thin and sharp 
on the outer edges, and gradually thickening as they approach their bases. These 
expansions consist of two distinct series of layers, deposited on the ventral and dorsal 
surfaces, and exhibit impressions which, as M. Deshayes remarks, are probably 
attributable to the presence of a vascular system in the substance of the mantle. 
It will be seen from the foregoing description that e/optera presents a much closer 
analogy with the Belemnites than that exhibited by Belosepia. The open semiconical 
cavity of the latter, in its typical form, nearly resembles the sheath of the Sepion ; 
but the lamine, both in their mode of arrangement and in their large siphoniform 
openings, present the first indications towards the phragmocone of the Belemnite. 
In the aberrant form, Pelosepia compressa, both the sheath and the laminz recede 
a step further from the Sepion type, and prepare the way for, and in fact connect 
Belosepia with, Beloptera. In this genus a still nearer approach to Belemnite 
appears; the wide, open, but shallow sheath of the Sepion, with its siphonless 
and nearly parallel laminz, is lost, and is replaced by an entire conical sheath, con- 
taining regular transverse septa perforated by a siphuncle, and exactly corresponding 
with the sheath and phragmocone of the Belemnite. The fold of the Belosepion, 
formed by the retroflexion and lateral enlargement of the ventral paries of the sheath, 
largely developed in the typical form, disappears in Peloptera, and is represented by the 
lateral expansions which characterise that genus, and which, greatly reduced in size 
in Beloptera Levesquei, lead directly into the simple sheath of the Belemnite ; while the 
strongly inflected rostrum of the Belosepion assumes the form of a somewhat conical 
mass, and thus prepares the way for the elongated and regularly conical guard of 
Belemnite. 
