22 EOCENE MOLLUSCA. 
OrpER—DIBRANCHIATA. Sts-orperR—DECAPODA. 
Tribe—OIGOPSID®. 
lst Family—BELEMNITID2. 
The Belemnitide, 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, Conoteuthis, Belemnitella, 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 Belemnitide, 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 Spzrw/a, with whose spiral discoidal 
shell they present so little analogy. 
It appears to be desirable that, for the present at least, the Belemnitide 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 lme in the form of a cone, or ina spiral or subspiral line. As 
thus enlarged, the Belemnitidz 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 Be/optera, Belemnosis, 
and Belosepia, leading to Sepide; Belosepia and Spirulirostra to Spirulide; and 
Conoteuthis to Teuthide. 
