i8 4 PALEONTOLOGY 



short distance into the guard, which probably began to 

 be formed as early as the phragmocone. 



The phragmocone when found alone is apt to be 

 mistaken for an Orthoceras, but it can be distinguished, 

 (i) by the marginal position of the siphuncle, and (2) by 

 the fact that the growth-lines (conothecal since] , which 

 form its only ornament, curve forwards on the dorsal 

 region in correlation with the presence of a pro-ostracum 



(Fig. 53. 4 



The chief features serving to distinguish speciesiamong 

 belemnite-guards are (i) 'the general shape, (2) the dis- 

 position of grooves on the surface, (3) the depth and 

 apical angle of the alveolus. The two main shapes are 

 the lanceolate and the hastate : in the former the diameter 

 is constant for the greater part of the length, and the 

 posterior end is conical; in the latter the diameter in- 

 creases from the front end for some distance backwards. 

 In cross-section either form may be circular, compressed 

 (laterally) or depressed (dorsi-ventrally) : to distinguish 

 between the latter in the absence of the phragmocone, it 

 is sufficient to notice the plane of symmetry of the 

 alveolus which is the median plane. The alveolar angle 

 varies between 12 and 32. 



A few belemnite-guards are quite free from grooves ; 

 others have a number of short apical grooves ; but the 

 majority have either a median ventral groove or a pair of 

 dorso-lateral grooves. 



The classification of belemnites is in a confused and 

 unsatisfactory condition. Only morphological classifi- 

 cations have been proposed, except for limited groups of 



