244 THE HABITAT Of THE EURYPTERIDA 



of eurypterids. Moreover, we can not understand why one species 

 of the Wenlock recurs in the Upper Ludlow, but does not occur in 

 the beds of intermediate age, although there are many such with 

 good marine molluscan faunas and even with fragments of other 

 eurypterid species. Such a perpetuation, to repeat, would be impos- 

 sible if the eurypterids were not having a continuous existence in the 

 sea. But their remains are at all times spasmodic in appearance, 

 being altogether wanting in certain horizons, especially where the 

 typical marine fauna is abundant. The fact that they occur in a 

 given band which, when traced even a short distance laterally, 

 shows no lithological change, but only an absence of eurypterids, 

 indicates that migrations along shore were non-existent; while the 

 fact that new species and even new genera appear at horizons 

 far separated from underlying and overlying eurypterid horizons 

 seems to deprive " marine" eurypterids of ancestors or descend- 

 ants, while to account for a marine Stylonurus ornatus in the 

 Wenlock of Lanarkshire and in the uppermost Ludlow of the 

 Pentland Hills, is not within the inventive powers of the author. 

 But, on the other hand, the conditions of bionomy in rivers are emi- 

 nently satisfactory to account not only for the persistence of a species 

 for a long period of time without morphological modifications of 

 specific rank, but also for the development of new species and genera, 

 and for their sudden appearance. This takes place, because they 

 have been developing either in other river systems, whence they have 

 migrated to the headwaters of the river at the mouth of which their 

 remains are found, or because they have been traversing a great dis- 

 tance in longitude, automatically suffering specific variation in their 

 progress. In this way, would I account for the anomalies in dis- 

 tribution just dwelt upon (see also p. 203 et seq.). 



Of this Ludlow fauna there still remain four species to be con- 

 sidered. There are three species of Eusarcus which may be taken 

 up at the same time: E. scorpioides, E. obesus, and E. raniceps. The 

 last species may be quickly dismissed, since it is represented by a 

 single specimen showing only the carapace and a part of the abdo- 

 men, enough, indeed, to place the individual generically; but specific 

 comparisons are impossible. E. scorpioides is represented by one 

 almost entire individual, a large, robust form in many respects similar 

 to E. scorpionis from the Bertie waterlime. The length and width 

 of the appendages, the number and disposition of spines thereon, the 

 ratio of length of carapace to the remainder of the body, and the 



