206 Annals oj the South African Museum. 



rapidly from about the sixth segment ; the rings of the axis are 

 nodular at their extremities, and each ring probably bore a strong 

 median spine. The pleurae are abruptly bent downwards at about 

 one-third of their length from the axis ; they are deeply grooved, and 

 the last six at least are produced into sharply recurved spines. 



Tail triangular, strongly arched, rather wider thaii long. Axis 

 very prominent, especially at its extremity, produced beyond the 

 margin, and terminating bluntly ; it bears nine ribs, the last three or 

 four of which tend to become obsolete. Lateral lobes steeply in- 

 clined, with five wide shallow grooves separated by narrow rounded 

 ridges. Margin apparently entire. 



The complete specimen represented in fig. 6 shows only ten 

 segments to the thorax, and thus differs from the typical members 

 of the genus. 



Bemarks. — Salter's Fhacops africanus appears to include at least 

 four distinct species, and unfortunately the original of his fig. 1 has 

 not yet been recognised with certainty. It seems, however, to be 

 identical with the specimens here described, and I have accordingly 

 adopted his name for this form. In the size and position of the eyes 

 and the character of the glabellar furrows my specimens do not agree 

 with Salter's description, but they closely correspond with his fig. 1. 

 For these characters he seems to have relied upon the head shov;n 

 in his fig. G, which is now in the British Museum of Natural 

 History. This, however, is certainly distinct and possesses neither 

 the genal nor occipital spines which he describes.''' 



Similarly, Salter's account of the tail seems to have been based 

 chiefly upon the specimen represented in his fig. 4 (11,286 in the 

 Museum of the Geological Society) and upon that represented in his 

 fig. 9 (now in the British Museum),! but these are quite distinct 

 from each other and neither of them agrees with the tail of my 

 complete specimen. The tail w^iich he describes — but does not 

 figure — under the heading Phacops, sp., No. 1 seems to resemble 

 very closely the tail of my specimens. 



For reference I may add that the originals of Salter's figs. 

 2, 3, 4, 5, and 8 are now in the Museum of the Geological Society 

 of London ; and the originals of figs. 6, 7, and 9 are in the British 

 Museum of Natural History. 



Affinities. — The form with which Phacops africanus is most likely 



* In the British Museum there are several other heads which present precisely 

 the same characters as Salter's fig. 0. 



t The original of his fig. 2 does not now bear any traces of the marginal spines 

 which he describes and figures. 



