TRIARTHRUS RHINETONENSIS. 71 



backwards and outwards to the posterior margin, which it cuts just within the genal 

 angle. Fixed cheeks narrow in front, expanding to about two-thirds the width of 

 the glabella behind; frontal limb narrow. Free cheeks very narrow, with the 

 genal angles produced into slender spines which bend a little outwards at their 

 origin and then curve backwards, extending to about the ninth or tenth thoracic 

 segment. Margin consisting of a rather broad and shallow furrow with a narrow 

 raised edge outside it. 



Thorax of fourteen segments, the axial rings of the last two fixed to the tail. 

 Axis considerably wider than the pleurae in front, about equal in width behind, 

 each segment bearing a median tubercle (not visible on internal casts) ; the twelfth 

 segment provided with a long median spine directed backwards. Pleurae bent 

 slightly downwards very near to the axis ; extremities of the early segments 

 obliquely truncate, of the later segments rounded ; pleural grooves deep and broad. 



Tail small, rounded, wide in proportion to its length, with a narrow entire 

 margin. Axis forming about one-third of the total width, reaching the posterior 

 margin, divided into two rings and a terminal portion. Lateral lobes with two 

 distinct furrows faintly interlined. 



Dimensions. — Length about 20 — 30 mm. 



This is the species named by Mr. Raw Triarthrus shinetonensis. He gives no 

 description or figure, but he has very kindly supplied me with a photograph. He 

 informs me also that the segments in front of the twelfth have fairly prominent 

 spines which seem to diminish anteriorly. I have not seen these myself, but in 

 other species have found that tubercles on the axis are often really the bases of 

 spines (see for example Ctenopyge bisulcata, p. 82). 



The long thoracic spine is not always shown on internal casts, and, as is the case 

 in so many other forms, the tubercles may be indistinct or absent. With regard 

 to the limits of the thorax, in almost every specimen there is a break at the twelfth 

 segment, the thirteenth and fourteenth segments lying, with the tail, at a slightly 

 different level from the rest of the body. The pleurae of these segments are usually 

 separated from the tail, but the axial rings appear to be united. There is some- 

 times, however, a distinct trace of an articulating surface between the fourteenth 

 segment and the tail. 



Triarthrus shhwtonensis is evidently an intermediate form allied on the one hand 

 to Parabolinella and on the other to Triarthrus. It resembles the former in the 

 general shape of the head, the truncate glabella and the presence of genal spines ; 

 it approaches the latter in the narrowness of the frontal limb and of the free cheeks, 

 in the width of the thoracic axis and the shape of the pleurae. It is not a typical 

 form of either genus, but it appears to be more closely related to Triarthrus than to 

 Parabolinella. 



Nevertheless, the species with which it is most likely to be confounded is 



