200 BKLEMNITI1KK 



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

 apical part of the phragmocone. " The phragmocone is not a 

 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 laminae of growth. 

 Professor Owen describes the fibres, in specimens from Chris- 

 tian Malford, as of a trihedral prismatic form, and one two-thous- 

 andth of an inch in diameter. These fibres are disposed con- 

 centrically around an axis, the so-called apical line, which extends 

 from the extremit} 7 of the phragmocone 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 mucronata 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 disap- 

 pears. The surface of the conotheca is marked by lines of 

 growth, and, according to Yoltz, 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 stria- 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-ostracmn in the family Bclcm- 

 nitidde. 



A. In many Belemnites the extension of the conotheca, seems 

 to run out in one simple broad plate, as in />. haxfatus, from 

 Solenhofen (lio-. if).'}). 



B. In Belemnites Puzosianus, d'Orbigny. the pro-ost raenm is 

 very thin, and apparently horny or imperfect ly calcified in the 



