BELBMNITID.3E. 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 regularlj^ placed and separated by cavities. The rostrum is 

 thick, turned towards the back ; the wing-like extensions of the 

 shell are chalky. 



Family 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 siphimcle. 



2. A guard or rostrum inore or less extensively enveloping the 

 apical part of the phragmocone. ■•' The phragmocone is not a 

 chambered body made to fit into a conical hollow previousl^^ 

 formed in the rostrum, as some have conjectured, but both the 

 rostrum and cone gi'ew 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 laminae of growth. 

 Professor Owen describes the' fibres as of a trihedral prismatic 

 form, and one two-thousandth of an inch in diameter. These 

 fibres 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 BelevmiteMa 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 maybe 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 striae 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 famih^ Belemnitidte. 



