308 BRITISH GRAPTOLITES. 



they are always somewhat stiff in appearance, and are generally arcuate in form 

 with the convexity upward. The bases of these spines are relatively broad, and 

 have the appearance of being formed as prolongations of the combined thickened 

 selvages of the whole of the denticle formed by the apertural and ventral edges of 

 the theca. In general, these spines are about equal in length to the breadth of the 

 polypary itself. In the proximal region they are directed downward but gradually 

 alter their direction, first to a horizontal one in the middle of the polypary, and 

 eventually to an ascending one towards its distal end. 



In addition to the apertural spines, the polypary always appears to possess a 

 series of septal spines. These originate from the mid-line of the polypary in 

 a direction at right angles to the direction of the apertural spines, and are con- 

 sequently best seen in the scalariform view, in which, owing to the direction of 

 compression, the apertural spines are hidden, or so foreshortened as to be rarely 

 visible. The septal spines occur both on the obverse and reverse aspect of the 

 polypary, and in both cases apparently originate at or near the bases of alternate 

 theca3. As a rule they are larger and stiffer than the apertural spines, and usually 

 take the form of blind spurs with broad bases. 



Owing to the presence of these two sets of spines, the appearance of the 

 polypary varies according to the direction of compression. In the bi-profile 

 view the apertural spines alone are seen, and may appear to be single or 

 paired. In the scalariform view the septal spines alone are usually visible, and the 

 tips of the apertural spines are presented in addition only in rare cases. In the 

 sub-scalariform view both sets of spines are visible in whole or in part, and 

 the ventral margins of the polypary have then a most remarkably spinose appearance 

 throughout. 



The thickenings of the angles and edges of the test in this genus are mainly 

 confined to the apertural and ventral edges of the thecas and the bases of the 

 septal spurs ; but there is occasionally seen a well-marked longitudinal thickening 

 bordering what appears to be the outer line of the septum on the two aspects, 

 forming a pair of more or less continuous longitudinal ridges or strands (?) running 

 parallel with the central line of the polypary. 



Four species of Glossograptus and one variety have been recognised in the 

 British Isles, viz. : 



Glossog. cfr. ciliatus. 

 Glossog. Hincksii. 



var. fimbriatus. 

 Glossog. armatus. 

 Glossog. acanthus. 



