SUB-CLASS ii DIBRANCHIATA .v.r> 



This family, owing to its great diversity of forms and geological imjiortance, occu 

 foremost position among Belemnoidca, The shell may be cou-i-l. r. ,1 as the prototype oi th.it 

 in all Dibranchiates, since it lias all the component ji.-u-t.s l'ull\ .1. \ . loj,,-,), \\hi-reaa in other 

 groups some of these become atrophied. 



The shell of Bdemnitcs (Fig. 1237) consists of three fundamental portions: (1) A 

 calcareous piece, usually much elongated, and of >ul>-ryliiniri<'.il, conoidal, or fusiform 

 This is called the guard (rostrum, osselct, gain<\ >'</ /'/), and is excavated at its aut'iior 

 broad extremity into a conical cavity or alveolus. Within tin- alveolus is placed (2) the 

 phragvwcom. This consists of a conical series of chambers (loculi), the septa of which .in- 

 pierced at the ventral margin for the passage of the siphuncle. The phragmocone begin^ \s itli 

 a globular protoconch, and its last or anterior chamber is of comparatively large size. It i.s 

 invested with a thin proper wall (conotheca), which is prolonged forwards mi the <l<u>al >ile 

 into a more or less calcined plate called (3) the proostracum. This last corresponds to the 

 "pen" of living cuttle-fishes. There is evidence that its anterior margin is convex, hut it i.s 

 so extremely thin that it is never perfectly preserved, and like the phragmocone, i> \\ anting 

 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 proostraciun 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 out- 

 occupying about one-eighth of the circumference, and covered with very obliquely arched 

 lines in a 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 ; proostracurn 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 



