PARABOLINELLA TRIARTHRA. 69 



which is not always very distinct. Anterior pleurae with the fulcrum placed about 

 one-third of their length, or less, from the axis, the portion outside the fulcrum 

 bent slightly downwards and backwards, the ends produced into short points; in 

 the later segments the fulcrum becomes progressively less marked, and the ends 

 less and less pointed until in the last two or three they are completely rounded ; 

 pleural grooves narrow. 



Tail very small, width nearly three times its length, margin entire. Axis 

 forming about one-third of the total width, consisting of one ring and a terminal 

 portion, ending bluntly, not quite reaching the posterior, margin. Lateral lobes 

 marked by two furrows. 



Dimensions. — The large specimen shown in Plate VII, fig. -1, would, if complete, 

 be about 65 mm. long, and there is a cranidium of a considerably larger individual 

 in the Sedgwick Museum. A length of thirty or forty millimetres is, however, 

 more usual. 



The boundary between tail and thorax is not perfectly definite. The twenty- 

 first segment seems to be loosely attached to the tail ; the pleural portion easily 

 comes apart, but the axial ring is more firmly united. Sometimes the axial ring of 

 the twentieth segment also seems to be joined to the tail. 



The pits in the marginal furrow are seldom visible and the first pair of 

 glabellar furrows is often indistinct or obsolete. The median tubercles on the axis 

 are usually inconspicuous on internal casts and sometimes appear to be absent ; 

 they are better shown on external moulds. All these differences are evidently due 

 to the state of preservation. 



There is more danger of confusing this species with Triarthrus shinetonensis than 

 with any other species of its own genus. It differs from P. villiamsoni and 

 P. esssa, in the emargination of the front of the glabella as well as in the glabellar 

 furrows; from P. rugosa in the smaller number of glabellar furrows; and from 

 Triarthrus sJtiuetonrnsis in the much wider frontal limb. The large number of 

 segments distinguishes the thorax from P. williamsoni (with about fifteen), P. cxsa 

 (with sixteen), and Triarthrus sliinetomnsis (with fourteen) ; and from the latter it 

 is also separated by the absence of the long spine on the twelfth segment and by 

 the comparative weakness of the pleural grooves. The tail is much smaller than 

 in P. irillianisoui and P. c&sa, but is not unlike that of Triarthrus shinetonensis ; in 

 the last, however, there are two distinct rings on the axis, and the furrows on the 

 lateral lobes are stronger. 



The specimens shown in Plate VII, figs. 10 and 11 differ in some respects from 

 the typical form, but the differences are perhaps due to age. The glabella is more 

 nearly parallel-sided and more rounded in front, with the first pair of glabellar 

 furrows almost invisible ; the facial suture runs more obliquely inwards from the 

 anterior margin to the eye ; the pleural grooves are stronger, more like those of 



