BRITISH FOSSILS. 



the pleurae are bent downwards. The convexity of the body rings, 

 however, though considerable, is much less than half that of the 

 head. 



Tail semicircular, the axis is entire and convex ; it is marked 

 above, like the thorax joints, by a strong articular furrow, but has 

 no other visible segments ; it occupies fiilly one third the width of 

 the tail, in some specimens more, — and is surrounded by a distinct 

 furrow. The sides are convex to their edge, the upper furrow 

 strong and abruptly terminated ; and no others are visible. The 

 axis is smooth, the sides lineated parallel to the margin of the tail. 



Variations. — We have not a sufficient number of perfect speci- 

 mens to ascertain what may be the amount of variation in propor- 

 tionate width, &c. ; but it is evident that some have a longer and 

 wider axis to the tail than others, and this would probably accom- 

 pany a similar difference in the head and thorax ; fig. 4 shows a 

 specimen, full grown, in which the axis is considerably larger than 

 in figs. 2 or 5. 



Affinities. — We have already mentioned the close relation this 

 has with the American genus Triarthrus, and the relation is perhaps 

 the most intimate in those points in which they differ from the rest 

 of the Olenidce. Indeed were it not for Triarthrus, of whose 

 affinity with Olenus scarabmoides there can be no doubt, it would 

 have been very difficult to assign a systematic place to this minute 

 and anomalous Crustacean. It has neither the parabolic glabella 

 with its shallow parallel furrows, the long smooth eyes connected to 

 the glabella by an ocular ridge, or the numerous body rings of many 

 Olenidw, but in the shape of the pleurae, and in the short rounded 

 tail, Triarthrus agrees with it, and they are similar too in a pecu- 

 liar character quite anomalous in the group, viz., that the maxillary 

 portion or free cheek is so reduced in size and length, that the 

 facial suture ends on the external margin, and the posterior angle of 

 cheek is turned upward to meet it, and supply its place. Of course 

 in this case there can be no spine to the hinder angle, and thus 

 another usual character of the group is lost. The inflation of the 

 glabella, the minute eye, which does not seem to possess even the 

 usual covering lobe (very distinct in Triarthrus), and the few body 

 rings, 7 instead of 16,* fitted much better than Triarthrus for coiling 

 up, give so distinct a character that we conceive it to form a very 

 natural genus. 



* Hall says 13, but there are as many as 16, exclusive of 4 or 5 in the tail, in a fine 

 specimen presented to us by Dr. Bigsby. 



