190 Proceedings of the Royal Society of Victoria. 



segmentation, notwithstanding the adherent matrix ; (2) the 

 absence of any trace of a limb, or striated margin posteriorly or 

 laterally; and (3) the presence of the apical emargination. There 

 is, on the other hand, a definite thread-like margin round the 

 sides and hinder portion, which at once dispels the idea that the 

 plate might be a portion of some other organism ; and I think 

 that the lateral segmentation radiating outwards on both sides 

 places its identity, so far as the generalised systematic position is 

 concerned, beyond doubt, but a reference to some one of the known 

 genera is a more difficult task. Perhaps the easiest method of 

 arriving at a decision on this point will be by a process of 

 elimination. The characters, so far as they can be deciphered, at 

 once forbid the entrance of the fossil within the families of the 

 Harpedida?, Remopleuridye, Olenid^e, Conocephalidje, Calyu)enid8e, 

 ^glinidfe, Cheiruridfe, Encrinuridae, Didymenidte, Acidaspidfe, 

 Lichadidee, Phacopidfe, Proetida?, Trinuclidas, and Agnostidaj, 

 thus leaving the Asaphidte, Bi'onteidse, and Illfenidje to choose 

 from. 



In the Asaphida?, Asaphiis and Ogygia being the typical 

 genera, the caudal shield is often of large size, and in some 

 species of the former obscurely segmented, but in other Asaphi 

 both the axis and pleural segments are well defined. In Ogygia 

 the tail is wide transversely, with a wide striated limb. The 

 axis extends to the margin of the latter, whilst the pleural 

 segments are broad and flat. In Barrandia both axis and the 

 divisions of the pleurae are quite apparent, but in Stygina the 

 axis is, in fact, of the two, the more prominent; the pleural 

 segmentation is hardly to be noticed. 



In the Illsenidse, having for the type genus Ilkeniis itself, the 

 caudal shield is large in proportion to the thorax, seldom, if ever, 

 segmented — if the rudimentary axis be left out of consideration — 

 certainly very rarely on the plevirse, and always convex and 

 prominent. One of the few examples of segmentation on the 

 pygidium in Illcenus, known to me, is that of /. atavus, Eichwald* 

 and even in this instance it is very slight. 



In the Bronteidfe the tail is usually of large size in comparison 

 with the thorax, strongly sub-semicircular, or deeply fan-shaped ; 



» Holm, Mem. Acail, Imp. Sci. St. Petersbourg, 18S6, xxiii., t. 7, f. 4» 



