I 



666 STRUCTURE AND AFFINITIES OF TRILOBITES. 



Observations on Trilobites, founded on a comparison of their structure with that of living 

 Crustacea. By W. S. MacLeay, M.A. F.L.S. &c. 

 Trilobites were originally considered by Klein and others to be a particular kind of molluscous 

 shell with three lobes. This supposition, however, was afterwards abandoned as untenable, and 

 remained so until Latreille in the 7th volume of the Annales du Museum revived it and referred 

 the trilobitic fossils to the genus Chiton among the Mollusca. Latreille founded his argument on 

 the presumed absence of feet, and on the lateral edges of the body in several species having been 

 sub-coriaceous. It is evident, nevertheless, that these early inhabitants of the sea could not have 

 belonged to the sub-kingdom Mollusca, since they possessed compound sessile eyes and a distinct 

 labrum. They must, therefore, be assigned to the sub-kingdom Annulosa, in which we may 

 find many articulated animals which have compound eyes and a labrum very similar in structure to 

 those of Trilobites. Having a hard, shelly, apterous tergum and inconspicuous feet, the Trilobites 

 must have either belonged to the Order Chilognatha among the Ametabola, or to the Class of 

 Crustacea. But all the Chilognatha are terrestrial animals, and the obvious geological fact is, 

 that Trilobites resided in the sea. We must clearly therefore exclude them from the Chilognatha 

 and place them among the Crustacea, in which class it becomes now necessary to determine their 

 exact place. 



The Class of Crustacea, so remarkable above all other animals for the great variation of their feet, 

 both in number and form, is divisible into two groups ; those which have the eyes sessile or the 

 Edriophthalma of Leach, and those which have their eyes supported on moveable peduncles or the 

 Podophthalma of Leach. To the Edriophthalma the Trilobites clearly belong, and the question is 

 now reduced to determine merely whether they belong to the Amphipoda or those existing Cru- 

 stacea which do not undergo metamorphosis in their larva state, (among which I include not only 

 the Amphipoda of Latreille, but also his L/oemodipoda and Isopoda,) or whether they belong to 

 the Entomostraca or those existing Edriophthalma which do undergo a change of form in their 

 larva state. I conceive that the Trilobites will be found to differ in so many respects from both 

 the Amnhipoda and Entomostraca, that according to the present state of our knowledge, we must 

 allow them to form a distinct order, intermediate between the tribe Isopoda on the one side, and the 

 tribe Aspidophora on the other. 



Those circumstances which generally are reckoned most anomalous in the Trilobites are not in 

 reality so very extraordinary, since they may be detected in many Crustacea now existing. Thus the 

 trilobed form of the body occurs in Serolis and Bopyrus. The membranaceous or rather coriaceous 

 margin of the body, assumed by Latreille and others to exist in Trilobites, is to be found in the 

 female Cymothoce. In these last animals also, as well as in the female Bopyrus, we observe the 

 eyes to disappear as in many Trilobites. The compound eyes of Calymene are situated on the back 

 of the head but wide apart, and are composed of large facets. The same structure may be seen in the 

 male of Cymothoa trigonocephala, and many other Cymothoadce. The absence of antennae and the 

 rudimentary state of the feet, both occur in Bopyrus, the well-known parasite of prawns. In Sphe- 

 roma we have not only the onisciform body of Calymene, but also its property of rolling itself up 

 into a ball. In Spheroma also we find the large convex semicircular anal segment of Bumastus. 

 I think, therefore, that we can have no hesitation now in allowing the immediate affinity of the Tri- 

 lobites to Isopod Amphipoda, and more particularly to the Cymothoadce and that parasitical group 

 which is called Epicarides by Latreille. Indeed, if the Trilobites are once demonstrated to have 

 possessed articulated feet, it will be difficult to remove a male Bopyrus from the group. Here 



