8 INTRODUCTION. 



2clly. Tliat they exhibit the greatest analogies with the Isopodes, particularly with 

 Cj/mofJioa and Lit/ia. 



3dly. That the want of feet seems to be a necessary characteristic of their skeleton 

 formation, although this point still remains problematical. 



4thly. That these feet, if they existed at all, were most probably connected with the 

 branchial apparatus. 



An important result was evidently gained by the enunciation and establishment of these 

 four principles, and the consideration of the last assumption especially has given that 

 direction to future researches which is the proper result of a preliminary investigation. 



George Wahlenberg followed more closely the footsteps of Linnaeus than any of his 

 predecessors, and endeavoured to maintain his view respecting the afBnity of the Trilobites, 

 merely also changing Linnaeus's name of EntomoIifJtns into Entüwodrcic'ües. But as he was 

 no special zoologist, and as the groups of the Crustacea in general could not be very strictly 

 defined at that period, or their essential characters be readily distinguished from others, he 

 did not succeed in establishing such evidence as should be incontrovertible. He believed that 

 the Trilobites were most nearly allied to Limulus, and was inclined to transfer this similarity 

 also to the structure of the feet. The feet of the Trilobites, in his opinion, however, 

 were smaller than those of Limulus, and for this reason were absent in the fossils. In some 

 shields and rings he believes that he recognizes mere membranes that had been cast oflF, 

 there being no doubt that these animals must have cast their membranes in the manner of 

 the Articulata. In other respects he still leaves all the species in one genus, and describes 

 fourteen of them. 



The most perfect work of all is Al. Brongniart's 'Histoire Naturelle des Crustaces 

 Fossiles,' which appeared about a year after the publication of Wahlenberg's p.aper. It was 

 this work which first pointed to the generic differences of the Trilobites, exhibiting five 

 genera mostly well-distinguished ; the species were more accurately determined, and the 

 number then known was stated to be seventeen ; finally, there were here explained many 

 facts with regard to the geological history of Trilobites more elaborately than had been done 

 by Wahlenberg. 



Brongniart expresses the correct view with reference to the zoological relations, namely, 

 that the Trilobites are most nearly related to the Bmnchiopodes among the Crustacea, and that 

 the want of visible feet, as well as of visible antennae, accords very well with this. He 

 does not, however, dispute the analogy with the Impodes so distinctly as the subject requires. 

 The importance and influence of this excellent work on our knowledge of the Trilobites was 

 exhibited immediately after its appearance, since M. Schlotheim felt himself obliged to publish 

 an addition on this subject as a supplement to the former scanty results of his 'Treatise on 

 Fossils ;' and in this supplement, in which he gave an extract from Brongniart's work, 

 together with a description of some new species, the number of all the known species, 

 including three which are unsatisfactorily described, amounts, according to his enumeration, 

 to twenty-nine, from which, however, we must omit three, as decidedly not belonging 

 to the family. ' 



