22 EOCENE MOLLUSCA. 



Okder—DIBRANCHIJTJ. Svb-okvek—DECAPODA . 



Tribe — O i g o p s i d m . 



1st Family — Belemnitid^e. 



The Belemnitida, the sixth family in the classification proposed by M. d'Orbigny, 

 consist, according to that author, of the genera in which the animal was provided 

 with an internal horny or calcareous shell, having at the posterior part air-chambers 

 superimposed in a nearly straight line in the form of a cone, and pierced on the 

 ventral part by a marginal syphon. The family is confined, according to that author, 

 to the three extinct genera, Conoteut/iis, Belemnitetta, and Belemnites. 



The two latter genera, however, do not appear to fall strictly within the terms of 

 the definition ; for the posterior parts of their shells consist, as is well known, of a 

 spathose guard, frequently of considerable size, the anterior extremity of which is 

 produced so as to form an alveolus for the reception of the phragmocone. If, as the 

 fact is, the genus Belemnites forms the typical genus of the Belemnitida, it would 

 surely be proper that those genera which, like the type, possess camerated siphoni- 

 ferous shells, terminated posteriorily by a calcareous guard, should, at all events, be 

 included in the family. It is difficult therefore to conceive on what sound principle 

 Spirulirostra and Beloptera, but more particularly the last genus, whose calcareous 

 remains present so striking a resemblance to those of Belemnites, instead of being 

 placed in this family, have been associated with Spirilla, with whose spiral discoidal 

 shell they present so little analogy. 



It appears to be desirable that, for the present at least, the Belemnitidse should 

 be extended so as to comprise all the genera in which the animal possessed an internal 

 horny or calcareous shell, with or without a terminal guard, but containing air- 

 chambers pierced by a ventral siphuncle ; whether those chambers were superimposed 

 in a nearly straight line in the form of a cone, or in a spiral or subspiral line. As 

 thus enlarged, the Belemnitidse will consist of the following genera : Belosepia, 

 Beloptera, Belemnosis, Spirulirostra, Conoteuthis, Belemnoteuthis, Belemnitella, and 

 Belemnites. 



Hereafter it may be desirable to form a division for the reception of genera in 

 which, as in Conoteuthis, the apex of the sheath is simple. 



All attempts at a linear arrangement are absolutely futile ; but it will be seen 

 that in this, as in every case where several genera are grouped together, the 

 family presents aberrant forms leading to other groups ; thus Beloptera, Belemnosis, 

 and Belosepia, leading to Sepidce ; Belosepia and Spirulirostra to Spirulida ; and 

 Conoteuthis to Teutkidce. 



