1866.] WOODWARD STRUCTURE OF XIPHOSURA, 35 



In Limuli from Solenhofen and in the living Limuli, of a head 

 composed of eight somites (one being thoracic), followed by five 

 coalesced thoracic somites bearing branchiae, and one or more 

 coalesced abdominal ones, to which is articulated a long and pointed 

 telson. 



In proposing to associate the Eurypterida and Xiphosura as two 

 suborders of the Merostomata of Dana*, I do not for a moment 

 overlook the fact that they form two very distinct although closely 

 allied groups. 



Notwithstanding the great difference in the form of the body, 

 concentrated and coalesced in Limulus, and elongated in Pterygotus, 

 the Merostomata offer perhaps even less diversity of structure than 

 do the Decapoda, the main featui-e in both being the same. Thus, 

 in the Decapoda Brachyura (Crabs) we have the cephalothoracic 

 segments concentrated in the highest degree, and the abdomen a 

 mere rudiment. In the Decapoda Macrura (Lobsters), on the con- 

 trary, the abdominal segments are not unfrequently more than twice 

 the length of the cephalothorax, and bulky in proportion. 



Such forms as Hemiaspis, Pseudoniscus, &c. may very well repre- 

 sent the Decapoda Anomura, being, like them, intermediate between 

 the long-tailed Lobsters and the short-tailed Crabs. 



Pew classes offer so remarkable an instance of longevity as the 

 Crustacea, and few orders can be compared to the Xiphosura for 

 persistency. The Jurassic forms appear to differ little, if at all, from 

 those of our own day ; and even those of the Carboniferous epoch 

 were at once recognized as belonging to the same family. 



Had Limulus represented a higher type of Articulata, it is hardly 

 conceivable that it could have existed so long, and apparently un- 

 changed. But it seems to have been one of those eccentric groups 

 that appear from time to time in the zoological series, which, branch- 

 ing out into a by-way of its own, is checked from further onward 

 progress ; but being possessed of tenacity of life and great power 

 of reproduction, it holds its ground whilst higher orders have been 

 modified or swept away. 



This is one very strong argument, to my mind, in favour of the 

 higher zoological position of Pterygotus — that, being extremely larval 

 in its anatomy, it consequently possessed the capacity for further 

 development, and so has been modified and disappeared, Eurypterus 

 Scouleri of the Coal-measures being its latest representative. 



It is not uninteresting to notice the appearance of Scorpionidae in 

 the Coal-measures of Silesia, a form of Arachnida which to this day 

 possesses a very remarkable resemblance to Pterygotus, both in its 

 general shape and also in the arrangement of its organs. 



But in Scorpio the respiration is aerial {i. e. by tracheae), whereas 

 in Limulus and Pterygotus it was undoubtedly aquatic {i.e. by 

 branchiae). 



This would, however, be a perfectly parallel case to the change 



* See my ' Monograph, of the British Fossil Crustacea belonging to the order 

 Merostoma,' Part I., in the Monographs of the Palseontological Society, for 

 1865. 1866. 



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