MUSEUM OF COMPARATIVE ZOOLOGY. 195 



an era in the history of the discussion of the zoological affinities and 

 analogies of the Trilobites. He brought together the history of what 

 had been done up to that time, and added the results of laborious and 

 profound studies of his own. Summarized, the result of his investiga- 

 tion is best given in the following general conclusion : — 



" The Trilobites xoere a peculiar family of Crustacea, nearly 

 allied to the existing Phyllopoda, ajyproaching the latter family 

 most nearly in its genus Braxchipus, and forming a link connect- 

 ing the Phyllopoda loith the P.ecilopoda." 



1870. Mr. E. Billings's discovery of an individual of Asaphus 2^laty- 

 cephalus from the Trenton limestone, with traces of the appendages 

 beneath the dorsal shell, affords the first evidence of the presence of ar- 

 ticulated ambulatory appendages in the Trilobite. He homologized the 

 Trilobite with Limulus and added materially to the knowledge of its 

 structure by his discover}-. 



. Mr. Henry Woodward strongly supported Mr. Billings's in- 

 terpretation of the parts found in the Canadian Asaphus. 



1872. Dr. A. S. Packard accepted Burmeister's classification of the 

 Trilobite with the Branchiopoda, and, from the discovery of Mr. Bil- 

 lings and Mr. Henry Woodward, homologized it directly with Limu- 

 lus, adopting the following arrangement under Branchiopoda : — 

 1st Order, Cladocera. 2d Order, 3ferostomata ; Suborder, Xipho- 

 sura ; Suborder, Eurypterida. 3d Order, Trilohita. 4th Order, 

 Phyllopoda.* 



1873. M. Alph. Milne Edwards states that, notwithstanding the 

 small number of species of this group {Limulus, etc.), the zoologist 

 ought to consider them as constituting a particular class intermediate 

 between the Crustacea and Arachnida.f 



1877. C. D. Walcott illustrated sections of the manducatorv appa- 

 ratus, branchise, etc., and placed the Trilohita, Xiphosura, and Eury- 

 pterida as orders in the legion Merostomata, and under the sub- 

 class Gnathopoda. 



1879. Dr. A. S. Packard formed the subclass PalcBOcarida to em- 

 brace the orders Merostomata and Trilohita, the former order includ- 

 ing the Xiphosura and Eurypterida as suborders. 



From the time of Ch. Mortimer to the date of Mr. Billings's dis- 

 covery, the Trilobite was homologized with Limulus on the characters 

 presented by the dorsal shell. That these were variously interpreted by 

 naturalists is shown by the varying views of Linnaeus, Burmeister, 



* Boston Soc. Nat. Hist. t Ann. des Sci. Nat., Tome XVII. p. 56. 



