204 Aunah of the South African Museum, 



Thoracic segments with a prominent axis bearino- a strong median 

 spine. 



Tail triangular, somewliat wider than long (if we exclude the 

 terminal spine), produced into an upturned luucro which is about 

 half as long as the rest of the tail. Axis prominent, marked with 

 eight distinct rings, terminates obtusely. Lateral lobes arched down- 

 wards, bearing five ribs, exclusive of the articular ridge, separated 

 by deep, broad furrows ; the last two ribs are short and rather weak ; 

 all the ribs end just before the margin, and the first four or five are 

 duplicated, showing clearly the original segmentation of the tail. 



Bemarks. — The head and tail on which the description is l)ased, 

 were found at Gamka Poort, and they are both, together with 

 another fragment of a similar head, in the same kind of matrix — 

 a matrix which is diS'erent from that in which any of the other 

 trilobites from this locality occur. The tuberculation of the head 

 resembles that of Encruiurus cnsta-galli, and the tail is similar in 

 structure to that of the same species. These considerations lead 

 mc to infer that both head and tail belong to a single species. The 

 thoracic segment clearly belongs to the same animal as the tail. 



Affinities. — -The species is evidently closely allied to Encri)iaru!i 

 cvista-galli, described by Dr. H. Woodward in 1873, and may indeed 

 be identical with it. It is unfortunate that Dr. Woodward's speci- 

 men shows nothing of the head except the cheek, and that my 

 specimens of the head show no part of the thorax. Most of the 

 specimens, however, which I have referred to Phacops arhntcus 

 seem to have belonged to a larger ajm}nQ,lth.a.n E no in urns crista - 

 gain ; the tail also, besides being larger, is much broader in propor- 

 tion to its length than in that species. The tail, moreover, is only 

 slightly tuberculate ; but it is an internal cast, and the internal cast 

 of E. crista-galli does not show the tubercles very distinctly. 



On the whole, ihen^ P]iacoi)S arbiUcus is probably a distinct 

 species, but it evidently belongs to the same genus ; and as it is 

 clearly a Pliacopa (using the generic name in its wider sense), we 

 may conclude that E. crista-galli is a Pliacops also. Dr. Woodward, 

 who has seen the specimens, agrees that this conclusion is probably 

 correct. 



- Some of the specimens of the glabella are not very strongly 

 tuberculate, and then they present a general resemblance to 

 Pliacops africanus ; but they may be distinguished by the more 

 strongly marked anterior glal)ellar furrows and the oblique position 

 of all three pairs. 



Localitics.-~(Ai\n\ki\ Poort (-iO, 61, 62, 64, 68). 



