A REVIEW OF THE TRILOIUTES. 277 



tain extent at least, to roll themselves up into a ball. This property 

 amongst crustaceans, is shared by the terrestrial oniscithe, and by 

 several marine genera ; notably by splutrowo. a genus of small isopo- 

 dous crustaceans inhabiting the Baltic, the Mediterranean, and other 

 seas. These marine isopods possess, however, peculiar swimming 

 appendages attached to their caudal extremity : a contrivance of 

 which the trilobite was apparently destitute. 



in accordance with the views of Bttrmeister, the place usually 

 assigned to the trilobites at the present day, is amongst the phyllo- 

 pods : or, at least, in the section brnnchiopoda. With certain proba- 

 bilities in favor of this distribution, there are yet many considerations 

 against it. A more or less constant motion of the branchial feet 

 would seem to be almost essential to the economy of the branchio- 

 pods ; but, in the case of the trilobites, a function of this kind can 

 hardly be reconciled with the rolled up condition in which so many 

 m are found. If, as may be reasonably inferred, this condi- 

 tion were assumed as a protection under the influence of fear, it 

 would probably be retained by the animal for a considerable time. 

 Amongst existing branchiopods, not one appears to have the power 

 of thus contracting itself into a ball ; whereas, amongst the isopods, 

 both terrestrial and marine, the property is almost universal. The 

 shell again, in branchiopodoii3 crustaceans, if present at all, is de- 

 licate and fragile, and scarcely to be compared in any way with that 

 of the trilobites. Finally, tbe minute size — and size may be here 

 legitimately considered as a not unimportant element in the inquiry 

 — the minute and often microscopic dimensions of the branchiopods, 

 together with their general conditions of existence, oiler further 

 points of dissimilarity. The trilobites were certainly as nearly allied 

 to the isopods as to the branchiopods ; and, at the same time, they 

 had certain strong analogies, if not homologies, with the limuli : in 

 the position and aspect of the large compound eyes, for instance : 

 in many characters of the shell ; and to a certain extent in size, and 

 possibly in mode of life. It seems advisable, therefore, to keep them 

 as a distinct order, and so to frame the classification of the Crusta- 

 cea generally, as to shew their relations to the isopods and phyl- 

 lopods on the one hand, and to the limuli or xiphosura on the 

 other. The chief difficulty is in the collocation of the latter order. 

 To place the limuli with the suctorial parasitic Crustacea, according 

 to a still frequently adopted system, is manifestly in 

 to all natural analogies. And, again, if we place them 



class, M a distinct subdivision, '.heir typical relations be- 

 come altogether lost. l> this, w proper placet Are 



