60 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [DeC. 20, 1871. 



(viz. Limulida, Eurypterida, and Trilobita) cannot at present be 

 established, perhaps may always remain doubtful," and that we 

 should therefore combine them under one collective name, " Gi- 

 gantostraca,^' jjlacing them " beside " the Crustacea, we lay ourselves 

 open to the grave charge of destroying an established system with- 

 out setting up a new one in its stead. 



Take away the Trilobita from the pedigree of the Ci'ustacea, and I 

 submit that one of the main arguments in favour of evolution to be 

 derived from the class, so far from being strengthened, is destroyed. 

 From what are the Crustacea of to-day derived? Are we to assume 

 that they are all descended from the Phyllopods and Ostracods — the 

 only two remaining orders whose life-history is conterminous with 

 that of the Trilobita ? Or are we to assume that the Arachnida 

 are the older class ? 



" If," as Pritz Midler well observes, " all the classes of the Arthro- 

 poda (Crustacea, Insecta, Myriopoda, and Arachnida) are indeed all 

 branches of a common stem (and of this there can scarcely be a 

 doubt), it is evident that the water-inhabiting and water-breathing 

 Crustacea must be regarded as the original stem from which the 

 other (terrestrial) classes, with their tracheal resj)iration, have 

 branched off" * (p. 120). 



The accompanying Table wiU probably express, more strongly 

 than words, my grounds for retaining the Merostomata and the 

 Trilobita also in the Crustacea ; but the latter, for the present, 

 distinct from the Xiphosura, although I have no objection what- 

 ever to consider them nearly related (ancestrally) ; and this re- 

 mark equally conveys my view as to the relationship between the 

 Xiphosura and the Euiypterida, and between the last of these 

 and the Arachnida, But, seeing that there is good evidence of 

 numerous forms of tracheated Arachnida as far back as the Coal- 

 measures, in which there is also evidence of Eurypterida still 

 existing, it seems impossible to pretend that the diverging-point is 

 reached at which the latter cast off its aquatic existence and 

 commenced its terrestrial phase as an Arachnid. Neolimulus, 

 again, is a true Limuloid form, and it occurs as far back as Ptery- 

 gotus, or nearly so ; and Hemiaspis (one of the few intermediate 

 forms met with presenting characters between the long- and the 

 short-bodied divisions of the Merostomata) occurs also in Silurian 

 beds with Pterygotus and Eurypterus. The Trilobita, from which 

 have branched out the Merostomata, only end in the Carboniferous 

 period ; whilst the Isopoda, with which I have ventured to compare 

 them, have been traced back as far as the Devonian. 



By placing in a tabular formf the sum of the characters of 

 each order side by side, we are the better able to comprehend the 



* ' Facts and Arguments for Darwin,' by Fritz Miiller, with additions by 

 the author. Translated from the German, by W. S. Dallas, F.L.S. 



t I gave in a table a comparative view of the characters of these four groups 

 in my Report on Fossil Crustacea read before the British Association in Edin- 

 burgh, August 1871 (Brit. Assoc. Eep. 1871, p. 53). Noticed in the Geol. Mag. 

 vol.viii. 1871,p.524. 



