CEPHALOPODA. 25 
3d. A thin calcareous layer, covering the whole of the inner surface and the 
terminal cavity of the sheath; and 
4th. A series of thin laminz or septa imposed one upon another, at first nearly 
vertically, but assuming gradually a horizontal direction, owing to the convergence, 
towards the origin of the radiated fold, of their ventral margms, which are nearly 
straight, and connected by a calcareous plate, forming the ventral surface of the 
sheath. 
The undulating impressions which appear within the sheath are strongly defined 
on the dorsal aspect, but become faint as they approach the ventral surface. M. Voltz 
has described these undulations as impressions of the sutures of the alveolus ;* while, 
on the other hand, M. d’Orbigny considers them to be lines of growth, and not marks 
of the chambers, which, he says, in fact only occupied one half of the cavity. They 
are, however, strictly analogous with the similar impressions found in the Sepion, and 
are formed by the margins of the lamin or septa. Beimg formed in succession as 
the new lamine are added, it is true that in that way they represent the progressive 
increase of the shell; but they are not true lines of growth. 
The extreme fragility of the lamine has not allowed of their preservation; but 
their remains occur, not unfrequently, towards the posterior extremity of the sheath, 
consisting of fine elevated lines, which traverse the whole circumference of the cavity, 
and are, in fact, the dorsal and lateral margins of the laminz adhering to the inner 
sheath. These lines are continued over the calcareous plate, which connects the 
ventral margins of the laminz ; and it is evident, therefore, that the laminz extended 
across the whole of the transverse area of the sheath. 
The ventral margins are always convergent towards the origin of the radiated fold ; 
and, consequently, the laminz within the terminal cavity slant in a direction opposite 
to that of the laminz within the sheath, inasmuch as that the cavity extends wholly 
below the origin of the fold. Owing to this, the arrangement of the chambers formed 
by the septa somewhat resembles that of the air-chambers in Spirulirostra, except that 
in the latter shell the plane of the septa is always at right angles with the axis ; while 
in the elosepion it is at an angle more or less acute as the septa approach to, or 
recede from, the point of convergence. In the cavity itself, the dorsal margins of the 
lamine are distant; but as they approach that part of the sheath which is immediately 
under the point of convergence, they are placed more closely to each other, and they 
again become distant as the laminze emerge from the cavity. Owing to the convergence 
of their ventral margins, the laminze, which as they emerge are nearly vertical, take a 
direction gradually more and more slanting towards the anterior extremity of the 
rostrum, until, on the shell attaiming its full growth, they assume a position nearly 
* The word alveolus is used by this author in its original meaning, and is applied to the chambered 
cone which Professor Owen has named the phragmocone. The term alveolus has been with greater propriety 
restricted by the latter gentleman to the cavity in which the phragmocone was lodged. 
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