TO THE EXISTING ARTICULATA. 35 



SECTION XV. 



A correct estimation of the characters exhibited by the Malacostraca will prove to us 

 at once that the Trilobites cannot, at all events, belong to this second principal division of 

 the Crustacea, for they have neither facetted eyes (see Sect. V), nor a common thoracic shell, 

 nor a constant number of (from five to seven) thoracic rings, which would necessarily belong 

 to them if the thoracic shell were wanting ; at least, the number could not be exceeded.* 

 This view, however, is likewise further confirmed by the absence of feelers with a horny 

 shell, by the enlarged, shield-formed head, by the absence of visible articulated equal feet, 

 and by the unequal numerical proportion of the abdomen, which is covered by one common 

 shield. Thus we disprove the aifinity of the Trilobites with the Impoda, and especially with 

 the genus Serolis, which was insisted upon by so man}' of my predecessors. In order to 

 manifest the incorrectness of such afiinity to every one, I have represented Serolis paradoxa 

 (Onisc. paradoxus, Fabr.) side by side with species of the Phyllopoda (Table II, Fig. 2), and 

 I think that the mere inspection of these different forms must convince every unprejudiced 

 person that the opinion of the aflänity of the Trilobites with Serolis cannot for a moment 

 be entertained. No single genus of the Trilobites has exactly the same numerical pro- 

 portion, nor, indeed, is there any other similarity with Serolis, excepting that which is 

 founded in the general class characteristics, and I must therefore, most decidedly, declare 

 myself against the arrangement of the Trilobites among the Malacostraca, being unable to 

 perceive a single reason in favour of such view. Indeed, even the moveable lateral lobes 

 at the thorax rings of many of the Isopoda cannot be compared with the lateral lobes of the 

 Trilobites : first, because they are moveable ; and secondly, because they belong to the leg, 

 properly speaking, and represent the modified hip of the latter, as I shall subsequently 

 prove ; I will here only remark that all the Isopoda, in which the moveable lateral lobes 

 are wanting, possess in lieu of them a fundamental joint on each leg above the hip, which 

 represents the rudiment of a lateral lobe. 



It can also be shown with as little difiiculty that the Trilobites have nothing in 

 common with Liniulus, excepting a very superficial resemblance. The absence even of a 

 separated head and thorax in this genus would render the afiinity impossible ; the hard, 

 powerful, horny feet, however, which have been so well preserved among the petrified Limidi 

 of the Jura formation, and which, therefore, we cannot doubt would be seen also in their 

 analogues of a more ancient period, are a still greater reason against it. The well-known 

 power in the Trilobites of doubling themselves up would have been a very unnecessary 

 gift, if they had had legs like those of the Limuli, since the latter are much too large to 

 admit of their being concealed when folded, and the animal is much too powerful to require 

 the protection afl'orded by that process. Nevertheless we must admit that the general form 

 of the cephalic shield, the absence of antennae, the position of the eyes, and the existence 



* Some Arthrosiaca, as also the Lamodlpoda, have only six thorax rings ; some Isopoda (Praniza) 

 only five, but no member of this group exhibits more than seven. Those exceptions, however, can 



rcachly be explained, and their origin traced. 



