PARABOLINELLA WILLIAMSONI. 05 



more or less outward, the posterior pleurae but little if at all facetted, and ending 

 in short falcate points which curve backwards. 



Tail rather large, broad, semi-circular, marginate. Axis narrow, about one- 

 fifth the total width, ending in a point which reaches the posterior margin, with 

 three definite rings, and a fourth somewhat indistinctly marked off from the 

 terminal conical portion. Lateral lobes with four grooves similar to the pleural 

 grooves, and fine intermediate impressed lines which are especially distinct near 

 the margin. Margin narrow, uniform in width, flat or slightly concave. 



An enlarged figure of the tail has been added, on Plate VII (fig. 1). 



Dimensions. — Length of the figured specimen 36'5 mm. 



This form is readily distinguished from Olenus longispina by the greater length 

 of the glabella and the much larger tail with the peculiar conical termination to the 

 axis. It closely resembles P. cxsa, but in that species there are three pairs of 

 glabellar furrows instead of two, and the conical termination to the axis of the tail 

 is absent or rudimentary. 



Most of the other species of Parabolinella have the glabella more truncate, and 

 as a rule they possess at least three pairs of glabellar furrows, but the first pair is 

 not always distinct. In most species also the tail is much smaller than in either 

 P. icilliamsoni or P. csesa. 



It should be noted that the presence or absence of median tubercles on the 

 thoracic axis or neck-ring is not a reliable character in this genus. Even amongst 

 the well-preserved Shineton Shale fossils many of the specimens of P. triarlhra 

 show no tubercles, whilst in others, and especially in external casts, the tubercles 

 are quite distinct. 



The specimen figured was named by Salter Olenus planti, and has since been 

 fully described under that name by Mr. F. R. C. Reed, who referred it to the 

 section Parabolinella, and pointed out the resemblance to Belt's Conocoryphe 

 williamsoni. Comparison with the specimens of that species in Belt's collection 

 in the British Museum has led me to conclude that the two forms are identical. 

 Unfortunately none of these specimens are complete. They include, however, 

 cranidia and tails. The tails are identical with that of the specimen here de- 

 scribed, and the only difference in the cranidia is that the frontal margin is some- 

 times more angulate. The facial sutures seem to have united along this margin, 

 and in some cases the anterior horns of the free cheeks have been broken off. 



The head-shield with attached free cheek from Craig-y-Dinas, referred to by Mr. 

 Reed, may possibly belong to this species. The specimen, however, is very obscure, 

 and the glabella appears to extend forwards to the anterior margin. 



Horizon and Localities. — Upper Lingula Flags : Moel Gron ; Dolgelly. 



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