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244 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 impossible 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 descendants, 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 eminently 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 distance in longitude, automatically suffering specific variation in their progress. In this way, would I account for the anomalies in distribution just dwelt upon (see also p. 203 et seq.).

Of this Ludlow fauna there still remain four species to be considered. 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 abdomen, 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