BELEMNITID. 45 
General characters those of the family. Doubtfully separable 
from Sepia. 
The principal character of the shell or sepiostaire, is the hood 
of chalky plates which covers the posterior end; these partitions 
are regularly placed and separated by cavities. The rostrum is 
thick, turned towards the back ; the wing-like extensions of the 
shell are chalky. 
Famity BELEMNITID &%. 
The shell of Belemnites consist fundamentally of :— 
1. A hollow cone, the phragmocone (ii, 19, 20), with a thin 
shelly wall, termed the conotheca, and which is divided by trans- 
verse septa, concave above and convex below, into chambers or 
loculi; the chambers are perforated near the ventral margin by 
a siphunecle. 
2. A guard or rostrum more or less extensively enveloping the 
apical part of the phragmocone. “ The phragmocone is nota 
chambered body made to fit into a conical hollow previously 
formed in the rostrum, as some have conjectured, but both the 
rostrum and cone grew together; the former was formed on the 
exterior of a secretive surface, and the latter on the interior of 
another secretive surface.’’—PHILLIPS. 
The rostrum is composed of calcareous matter arranged in 
fibres perpendicularly to the planes of the laminz of growth. 
Professor Owen describes the fibres as of a trihedral prismatic 
form, and one two-thousandth of an inch in diameter. These 
tibres are disposed concentrically around an axis, the so-called 
apical line, which extends from the extremity of the phragmo- 
cone to that of the rostrum. Indications of a thin capsule or 
formative membrane appear in some Belemnites investing the 
guard; in those of the Oxford clay it is represented by a granular 
incrustation; in some liassic species it appears in delicate plaits, 
like ridges or furrows; in some specimens of Belemnitella mucro- 
nata from the upper chalk of Antrim, it is in the form of a very 
thin nacreous layer. 
3. A pro-ostracum, or anterior shell, which is a dorsal exten- 
sion of the conotheca beyond the end where the guard disappears. 
The surface of the conotheca is marked by lines of growth, and, 
according to Voltz, it may be described in four principal regions 
radiating from the apex; one dorsal, with loop lines of growth, 
advancing forward ; two lateral, separated from the dorsal by a 
continuous straight or nearly straight line, and covered with 
very obliquely arched striz in a hyperbolic form, in part nearly 
parallel to the dorso-lateral boundary line, and in part reflexed, 
so as to form lines in retiring curves across the ventral portion 
nearly parallel to the edges of the septa. There were at least 
three kinds of pro-ostracum in the family Belemnitide. 
