SUB-CLASS II DIBRANCHIATA 595 
This family, owing to its great diversity of forms and geological importance, occupies a 
foremost position among Belemnoidea. The shell may be considered as the prototype of that 
in all Dibranchiates, since it has all the component parts fully developed, whereas in other’ 
groups some of these become atrophied. 
The shell of Belemnites (Fig. 1237) consists of three fundamental portions: (1) A solid 
calcareous piece, usually much elongated, and of sub-cylindrical, conoidal, or fusiform shape. 
This is called the guard (rostrum, osselet, gaine, Scheide), and is excavated at its anterior 
broad extremity into a conical cavity or alveolus. Within the alveolus is placed (2) the 
phragmocone. This consists of a conical series of chambers (loculi), the septa of which are 
pierced at the ventral margin for the passage of the siphuncle. The phragmocone begins with 
a globular protoconch, and its last or anterior chamber is of comparatively large size. It is 
invested with a thin proper wall (conotheca), which is prolonged forwards on the dorsal side 
into a more or less calcified plate called (3) the proostracum. This last corresponds to the 
*‘nen”’ of living cuttle-fishes. There is evidence that its anterior margin is convex, but it is 
so extremely thin that it is never perfectly preserved, and like the phragmocone, is wanting 
in by far the greater number of specimens. 
Notwithstanding the fragmentary condition in which the proostracum invariably occurs, 
it is possible to reconstruct its outlines from the peculiar conothecal striae, or markings of the 
membranous substance with which it is invested. The conotheca is made up of three very 
thin superimposed laminae, the outermost of which usually shows the markings alluded to 
most distinctly (Fig. 1237, C). The conical surface of the phragmocone and proostracum is 
divided by Voltz into four principal regions radiating from the apex: A dorsal area, including 
all the space between two straight lines called the asymptotes, which extend from the apex of 
the cone as far as the aperture. This area occupies about one-fourth of the circumference, and 
is marked with loop lines of growth convex toward the front. On either side of the dorsal 
area and separated from it by the asymptotes is a lateral or hyperbolic area, each one 
occupying about one-eighth of the circumference, and covered with very obliquely arched 
lines ina hyperbolic form. The ventral area is covered with numerous transverse striae, of 
which there are many on each alveolar chamber, and they are closer together the nearer they 
are to the apex of the phragmocone. The striae of the dorsal area are less numerous than 
those of the rest of the shell, and usually less pronounced, being sometimes imperceptible. 
“The guard of Belemnites consists of prismatic calcareous fibres, which are directed 
perpendicularly to the surface, and radiate in all directions from an axial line, which is not 
strictly central, but is somewhat nearer the ventral than the dorsal side. The growth of the 
guard is effected by the deposition of successive conical layers or sheaths, which are secreted 
over the entire surface, but are thickest behind, and become gradually attenuated in front. 
The surface of the guard is smooth ; or may be wholly or partially granulated or wrinkled ; 
or, again, may be marked with branched vascular impressions, which are especially con- 
spicuous on the ventral side. In many cases a well-marked groove—the ventral furrow—runs 
from the edge of the alveolus backwards on the ventral side, extending for a short distance 
only, or reaching to the point of the guard (Fig. 1241, C). The apical portion of the guard 
often shows two symmetrical grooves (the dorso-lateral grooves) which diverge slightly and 
become shallower as they extend forwards, and which mark the dorsal side of the shell” 
(Nicholson). 
As shown by vascular impressions on the rostrum, the shell of Belemnoids was completely 
enveloped by the mantle. Natural casts of the animal occurring in the English Lias (Figs. 
1237 B, 1250) exhibit an elongated form of body, contracted anteriorly, with a small head 
surrounded by ten equal arms. An ink-sac is present, and the arms are provided with hooks. 
The maximum size attained by Belemnitoids is between 2 and 2°5 metres. 
Aulacoceras, Hauer (Fig. 1238). Guard elongated, clavate, contracted anteriorly, 
thickened in the posterior third, and pointed at the tip; composed of concentric, 
loosely superimposed lamellae. Each side marked by a deep broad lateral groove 
reaching from the tip as far as the anterior alveolar margin, Phragmocone at least 
‘twice as long as the rostrum, slowly increasing in width anteriorly, ornamented 
externally with raised longitudinal lines, which are crossed on the dorsal side by a 
transverse series, convex toward the front; closely resembling Orthoceras. Septa 
rather distantly spaced ; siphuncle marginal, thin; proostracum unknown, Guards 
of this genus are rare, but detached phragmocones not uncommon. Upper Alpine 
Trias. 
Atractites, Gumb. Like Aulacoceras, but guard large, smooth, and without lateral 
furrows. Phragmocone either smooth, or with fine asymptotic lines, and dorsal area 
marked with extremely fine growth-lines, convex toward the front. Guards and 
