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 laminae 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 margins, 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 arc 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 laminae or septa. Being formed in succession as 

 the new laminae 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 laminae 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 laminae adhering to the inner 

 sheath. These lines are continued over the calcareous plate, which connects the 

 ventral margins of the laminae ; and it is evident, therefore, that the laminae 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 laminae within the terminal cavity slant in a direction opposite 

 to that of the laminae 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 Sjriralirostra, except that 

 in the latter shell the plane of the septa is always at right angles with the axis ; while 

 in the Belosepion 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 

 laminae 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 laminae emerge, from the cavity. Owing to the convergence 

 of their *ventral margins, the laminae, 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 attaining 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. 



'4 



