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 siplnmclc. 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 Belemnitc. 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 laminae, radiating from 

 the apex of the cone, and so arranged, that the central laminae are in a 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 laminae 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 Beloptera 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 laminae, 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, Belosepia compressa, both the sheath and the laminae 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 laminae, 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 Beloptera, 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. 



