ANATOMY AND RELATIONS OF THE EURYPTERIDSE. 527 



since the scale markings, the presence of which has caused fragments of Glyptoscorpius to. 

 be referred to Eurypterus, are, as I have mentioned above, widely distributed among the 

 Arachnida. 



I have summarised the main ideas in the following table. A is the intermediate form 

 between Limulus and Eurypterus suggested above. I derive the Scorpions from some 

 little way up the Limulus stem, on account of some peculiarities in the anatomy of their 

 soft parts, which tend to separate them from the other Arachnids, and ally them to 

 Limulus. At (a) on the Eurypterus stem, the genital operculum was already well 

 developed. 



Araneidse. 



Pbalangium. 



Acaridae. 

 Thelyphonus. 



Eurpyterus. I I / Limulus 



Belinurus, &c. 



If the views set forth above prove to be correct, some changes will be necessary in the 

 classification and terminology of the groups involved. Arachnida, if the name is to have 

 any scientific meaning, must either be limited so as to exclude the Scorpions, or the 

 Eurypteridse and Xiphosura must be admitted within its bounds. The latter is manifestly 

 the better course to take ; and the Xiphosura, Scorpionina, Eurypteridse, and Thelyphonina 

 will form sub-orders of about equal value. Whether the other groups of Arachnids — 

 Pseudo-Scorpions, Phalangidse, &c. — are also to be placed as sub-orders of equal value to 

 the four mentioned above is a question rather outside the sphere of this paper, and which I 

 hope to discuss later. The term Pcecilopoda, used first by M'Coy,* and used by Walcott 

 to include the Trilobites, Xiphosura, and Eurypteridse, must, if it be retained, be used for 

 the Arachnida plus the Trilobita. 



"Merostomata" has such a classic position, as including the Eurypteridse and Xiphosura, 



* Ann. and Mag., ser. 2, vol. iv., 1849. 



