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BRITISH FOSSIL CRUSTACEA. 



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 respiration, have branched off/ " l 



As regards the near relationship between the aquatic gill-bearing Xiphosura and 

 Eurypterida, and the terrestrial tracheated Arachnida, we must bear in mind that the 

 rich deposits of the Carboniferous period in Europe and America have yielded numerous 

 examples of both these types. Scorpions and Spiders, differing but little from existing 

 forms, flourished in the Coal-period, and Eurypterus, as well as Limulus, still survived 

 from Silurian times. 



While fully admitting the probability of a common ancestry for the Scorpionida and 

 the Eurypterida we cannot pretend to say 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 Lirauloid form, and it occurs as far back in time as 

 Pterygotus, 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 the Silurian strata with Pteryyotus and Eurypterus. 



The Trilobita, with a far more remote ancestry than 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 form the sum of the characters of each order side by side* 

 we are the better able to comprehend the extent to which they are capable of being 

 paralleled ; and certainly it seems as if the verdict were in favour of as near a relation- 

 ship existing between the Isopoda and Trilobita as that which undoubtedly does exist 

 between the Xiphosura and the Eurypterida. 



Pterygotus (fossil extinct), j 



1. Eyes sessile, compound. 



2. Ocelli distinctly seen. 



3. All the limbs serving as mouth-organs. 



4. Metastoma, large, bilobate, oval. 



5. Anterior thoracic 



segments 



bearing either 



branchiae or reproductive organs. 



6. Other segments destitute of any appendages. 



7. Thoracic segments non-anchylosed. 



8. Abdominal segments free and well developed. 



Limulus (fossil and living). 



1. Eyes sessile, compound. 



2. Ocelli distinctly seen. 



3. All the limbs serving as mouth-organs. 



4. Metastoma small, bifid. 



5. All the thoracic segments bearing either 



branchiae or reproductive organs. 



6. Other segments destitute of any appendages. 



7. Thoracic segments anchylosed. 2 



8. Abdominal segments anchylosed and rudi- 



mentary. 



1 ' Facts and Arguments for Darwin,' hy Fritz M uller, with additions by the author. Translated from 

 the German by W. S. Dallas, F.L.S. (p. 120). 



2 Free and movable in the very young state, and probably also in some of the paleeozoic species, 

 e ff., Bellinurus and Neolimulus. 



