72 BRITISH CAMBRIAN TRILOBITES. 



Parabolinella triarthra. A comparison of Plate VII, fig. 7 with Plate VII, fig. 13, 

 will show tliat there is a striking general resemblance between the two forms. The 

 glabella is almost identical, but in T. sliinetonensis it extends farther forwards, and 

 the first pair of glabellar furrows is obsolete, as is also the case, however, in many 

 specimens of P. triarthra. The free cheeks are much narrower in T. sliinetonensis 

 and the eye-lobe is longer; the genal spines bend slightly outwards at their origin, 

 whereas in P. triarthra they continue the line of the cheek-margin. 



The thorax is decidedly different. T. shinetonensis has fourteen segments, 7'. 

 triarthra when adult has twenty-one ; in T. shinetonensis a long spine directed 

 horizontally backwards springs from the axis of the twelfth segment. In T . 

 shinetonensis the axis of the thorax is relatively wider than in P. triarthra, though 

 not quite so wide as in T. bedci ; the pleural grooves are deeper and broader than 

 in P. triarthra. 



The tail is very similar in the two forms, but in T. sliinetonensis there are two 

 rings on the axis instead of one, and the grooves on the lateral lobes are stronger. 



It is scarcely necessary to enter into any detailed comparison with the other 

 British species of Parabolinella. It is at once distinguished from P. williamsoni 

 and P. csesa by its small tail, and from P. rugosa by the presence of only two pairs 

 of glabellar furrows instead of four. 



It differs from the type species of the genus, Triarthrus beclti, in the presence 

 of genal spines and in the smaller number of segments in the thorax and tail. 

 The glabella, moreover, is less rounded in front and does not reach quite so far 

 forwards, and there is an ocular ridge, which appears to be absent in T. becli. 



But the form most closely allied to the present species is Triarthrus spinosus, 

 Billings. 1 The general shape of the head is nearly the same, and the glabella and 

 free cheeks are similar. In both species the genal angles are produced into slender 

 spines, which bend outwards at their origin, and in both the thoracic axis bears a 

 long median spine. In T. sjnnosns, however, there is in addition, a long spine 

 arising from the neck- segment, and the thoracic spine springs, not from the twelfth, 

 but from the eighth segment. Billings states that he could not find in any specimen 

 more than thirteen segments in the thorax and four or five in the pygidium. 



Triarthrus canadensis. — Smith, 2 also has the genal angles spined, but the 

 specimen on which the species is founded is so imperfect that no useful com- 

 parison is possible. 



Horizon and Local it)/. — Shineton Shales : Shineton. 



1 Geological Survey of Canada. Report of Progress for the years 1853 — 56 (1857), p. 340. 

 — Geology of Canada. Geological Survey of Canada. Report of Progress from its Coninu ncement to 

 1863 (1863), p. 202, fig. 199. 



- Canadian Journal, new series, vol. vi (1861), p. 275, with woodcut. 



