Palaeozoic Arachnida of North America. 19 



more than a mere hypothesis. One had only to imagine that such 

 eurypterids as Slimonia acuminata or Eusarcus scorpionis acquired 

 the habit of hving in very shallow water and changed gradually 

 into scorpions. The only difficulty in the way of such change seemed 

 to lie in the difference of the respiratory organs of scorpions and 

 Eurypterids. But the ingenious explanation of the origin of lung- 

 books from gill-books through an insinking of the latter first brought 

 forward by Lankester and the subsequent observations of Brauer 

 on the embryos of scorpions tended to obviate even this difficulty. 

 The correctness of this interpretation is so apparent, that Lankester 

 has even ventured to express the hope of finding some day specimens 

 of Silurian scorpions which still led aquatic life and breathed by 

 means of gill-books. The absence of spiracles in the fossil Palaeo- 

 phonus, a negative character on which Lankester builds his hope, 

 may however be due merely to poor preservation. This seems to 

 be the more probable since none of the upper Carboniferous arachnids 

 of North America show the slightest trace of spiracles and yet there 

 cannot be any doubt as to their having led a terrestrial life. For a 

 while I thought that perhaps the eurypterids themselves had already 

 internal gill-books of the lungbook type, with spiracles somewhat 

 similar to those of the scorpion, and connecting the gill cavity with 

 the outside. In that case nothing but a change in function would 

 be required to transform them into the lung-books as we find them 

 in recent scorpions. One could imagine that the insinking took place 

 in aquatic arachnids as a protection against injuries on the one hand 

 and angainst too rapid drying in case of temporary exposure to air on 

 the other hand. I tried to^find support for my assumption not in the 

 absence of spiracles in remains of terrestrial arachnids, but in the 

 size and position of the gill-books in Eurypterids. Disregarding the 

 presence of gill-books in the second abdominal somite where in the 

 scorpion they are modified into combs, their size and position in the 

 following four somites correspond almost exactly with the size and 

 position of the lung-books. A mere impression of lamellae on the 

 concretion in the absence of an impression of spiracles could be 

 readily interpreted in either sence. Morever, the overlapping of the 

 gill-plates and still more the presence of narrow bands, of the same 

 width as the overlapping, in specimens with distended abdomen 

 reminded me strongly of the conditions in modern scorpions where 

 the abdominal sclerites are connected with each other by means of 

 soft membrane. However, after some search for an explanation 

 Professor Schuchert was able to convince me that what I was in- 

 clined to interpret as overlapping sclerites of the abdominal wall 



