680 



MOLLUSCA— CEPHALOPODA 



PHYLUM VI 



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

 phragmaconc. This consists of a conical series of Chambers (loculi), the septa of which are 

 piereed at the ventral niargin Ibr the passage of the siphuncle. The phragmaconc begins with 

 a globular protoconch, and its last or anterior chamber is of coniparatively 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 proostracmn. This last corresponds to the 

 " pen " 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 phragmaconc, is wanting 

 in by far the greater nnmber 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. 1314, C). The conical surface of the phragmaconc 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 extends from the apex of 

 the cone as far as the apertiire. This area occupies about one-fourth of the circumference, and 

 is marked with loop lines of growth convex toward the front. On eitlier side of the dorsal 

 area and separated from it by the asymptotes is a lateral or 

 hyperholic area, each one occupying about one-eighth of the 

 circumference, and covered with very obliquely arched lines 

 in a hyperholic form. The ventral area is covered with numer- 

 ous transverse Striae, of which there are many on each alveolar 

 Chamber, and tliey are closer together the nearer they are to 

 the apex of the phragmaconc. The Striae of the dorsal area 

 are less numerous than those of the rest of the shell, and 

 usually are less pronounced, being sometimes imperceptible. 



"The guard oi 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 sraooth ; or may be wholly or partially granulated 

 or wrinkled ; or, again, may be marked with branched vascular 

 impressions, which are especially conspicuous on the ventral 

 side. In many cases a well-marked groove — the ventral farrow 

 — runs from the edge of the alveolus backwards on the ventral 

 side, exteiiding for a short distance only, or reaching to the 

 point of the guard (Fig. 1318, C). The apical portion of the 

 guard often shqws 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. Well 

 preserved impressions of the animal in tlie English Lias (Figs. 

 1315, .ß; 1327) 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 Belemnoids is between 2 and 

 2-5 metres. 



Aulacoceras Hauer {Didyoconites Mojs.) (Fig. 1314). 

 Guard elongated, clavate, contracted anteriorly, tliickened 

 ^ in the posterior third, and pointed at the tip ; composed 



Upper Trias ; Rr)theistein7'riear of concentric, loosely superimposed lameliae. Each side 

 phragmacone"^. ^]i, Guard, ^i/,*? ^'^^^^^^ ^7 a deep broad lateral groove reaching from 

 t^ ^°';V'°" °iP^''"^''"'i'l""e «ii^e'i the tip as far as the anterior alveolar margin. Phragma- 



10 Show siphuncle aml siphonal \ -i . o ö 



funneis. cone at least twice as long as the rostrum, slowly increas- 



. , . ^^o 11^ width anteriorly, ornamented externally with 



raised longitudmal lines, which are crossed on the dorsal side by a transverse series 



Fig. 1314. 

 Aulacoceras reticulatum Hauer, 

 near 



Vlth aj 



ries, M 



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