MUSEUM OF COMPARATIVE ZOOLOGY. 195 
an era in the history of the discussion of the zoülogical 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 Trinoprres were a peculiar family of CRUSTACEA, nearly 
allied to the existing PHYLLOPODA, approaching the latter Jamily 
most nearly in its genus BRANCHIPUS, and forming a link connect- 
ing the PHxLLoPoDA with the PæCILOPODA.”? 
1870. Mr. E. Billings's discovery of an individual of Asaphus platy- 
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 discovery. 
Mr. Henry Woodward strongly supported Mr. Billings’s in- 
terpretation of the parts found in the Canadian Asaphus. 
1872. Dr. A. &. 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, Merostomata ; Suborder, Xipho- 
sura; Suborder, Æurypterida. 3d Order, Trilobita. Ath Order, 
Phyllopoda.* 
1873. M. Alph. Milne Edwards states that, notwithstanding the 
small number of species of this group (Limulus, ete.), the zodlogist 
ought to consider them as constituting a particular class intermediate 
between the Crustacea and Arachnida. t 
1877. ©. D. Walcott illustrated sections of the manducatory appa- 
ratus, branchiæ, etc., and placed the Trilobita, Xiphosura, and Hury- 
pterida as orders in the legion Merostomata, and under the sub- 
class Gnathopoda. 
1879. Dr. A. S. Packard formed the subclass Palæocarida to em- 
brace the orders Merostomata and Trilobita, the former order includ- 
ing the Xiphosura and Hurypterida as suborders. 
From the time of Ch. Mortimer to ihe 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 Linnwus, Burmeister, 
* Boston Soc, Nat. Hist. + Ann. des Sci, Nat., Tome XVII. p. 56. 
