16 W. S. MacLeay on the Structure and 



III. — 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.* 



Trtlobites 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 un- 

 tenable, and remained so until Latreille, in the 7th volume of 

 the ^ Annales du Museum/ revived it and referred the trilo- 

 bitic 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 w^e 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 Tri- 

 lobites 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 geo- 

 logical 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 ani- 

 mals 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 w^hich have 

 their eyes supported on moveable peduncles or the Pod- 

 ophthalma 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 

 Crustacea which do not undergo metamorphosis in their larva 

 state, (among which I include not only the Amphipoda of La- 



* Reprinted with permission from R. I. Murchison's valuable work on 

 the * Silurian System.' 



