BELEMNITES OF THE LIAS. 



49 



DIAGRAM 20. 



f3^ 



111 the first place, some traces of growth lines appear on the alveolar cavity, and thin 

 layers adhering to the conotheca, which indicate the terminal edge of the guard to have 

 been in the form seen sideways at /, dorsally at d, and in front at v (Diagram 20). Next 

 the conothecal surface itself shows the structure lines ; the dorsal aspect as in d , the side 

 aspect as in /', enlarged in /"; and the ventral aspect as in v (Diagram 19). The 

 points to remark are the bifurcation of the arched side lines in /" before passing over the 

 front in y" ; the numerous transverse strise in v are from 6 to 8 to each concameration ; 

 and the undulated outline which the phragmocone deprived of conotheca presents (Dia- 

 gram 1 9 v') The transverse striae in v' are not so numerous in the smaller parts of the 

 conotheca ; there are only very faint longitudinal medio-dorsal lines ; the siphuncle makes 

 no distinct appearance externally. The sides of the phragmocone are nearly straight ; it 

 is terminated by a distinct and rather large spherule. The angle is about 20° (19° to 

 23°) ; the cross section s nearly circular. The excentricity of the alveolar apex variable, in 

 some specimens very slight. A gentle retral wave in each septum on the siphuncular line. 



Belemnites apicicurvatus, Blainville. PI. VI, fig. 16. 



Reference. Belemnites apicicurvatus, Blainville, ' Mem. sur les Belemnites/ p. 76, 

 pi. ii, fig. 6, 1827. 



Guard. Elongate, compressed, smooth. Alveolar region compressed, expanding towards 

 the aperture ; apicial region convexo-conical, obhquely inclined or even bent towards the 

 back, marked by two latero-dorsal furrows, extending a short distance from the summit, 

 and two still shorter, terminal, latero-ventral grooves. Between these four grooves are 

 often sharp short plaits. In very perfect specimens the whole apicial region is minutely 

 ornamented by longitudinal striations, which near the apex are straight and even con- 

 tinuous, but elsewhere undulate and anastomose. The anterior part of the alveolar region 

 is sometimes roughly striated. Of the plaits at the summit, the medio-ventrahs often 

 very short and sharp. 



Sections show the apicial line almost straight, nearer to the ventral face ; the succes- 

 sively superposed laminae of growth very distinct, the fibres faint, oblique, and somewhat 

 curved near the apex ; the substance honey-yellow spar. The ventral region is generally 

 broadest, the sides of the alveolar portion are usually flattened. This species sometimes 

 cracks naturally along the lateral faces. 



