MAN'TJAL OE THF. MOLLUSCA. 



fessor Owen describes tlie fibres, in specimens from Cbristian 

 Malford, as of a tribedral prismatic form, and Toooth. of an inch 

 in diameter. Tliese fibres are disposed concentricdily around 

 an axis, a, the so-called apical line, ■wbicli extends from the 

 extremity of the pbragmocone to that of the rostrum. Indica- 

 tions of a thin capsule or formatiye 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 disappears. 

 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. Fig. 2, a, with 



H 



FiiT. 2. rip. 3. Fig, 4. 



loop lines of growth advancing forward; two lateral, 6, separated 

 from the dorsal by a continuous straight or nearly straight line, 

 and covered with very obliquely arched strise 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. 



