412 GEOLOGY. 



extent of the increase of the land area, but also to the extent that sea- 

 water was replaced by water bodies too salt or too fresh for the strictly 

 marine life. 



The importance of recognizing the fresh-water or brackish-water 

 faunas, if they really were such, lies in the fact that they did not con- 

 stitute a relief or a resilience from the restrictional movement of the 

 marine fauna, but intensified it. They were a feature of the reciprocal 

 expansional evolution of the land life in the broader sense of that 

 term which includes the life of the land waters as well as that of the 

 land itself. The life of the land and land-waters expanded and con- 

 tracted together in general, and in a phase opposite to that of the sea. 

 The fauna which brings up this interesting question is that of the Water- 

 lime to which we now turn. 



The advent of a remarkable crustacean fauna. — The Waterlime 

 beds represent a gradual return of the Salina basin to a condition hos- 

 pitable to life. The fauna contained in these beds is limited, but most 

 interesting and suggestive. In its dominant aspect it was a radical 

 departure from that of Mid-Silurian stage, in that most of the familiar 

 marine types were absent, while its signal feature was an abundance 

 of large crustaceans of types barely represented before. The most 

 characteristic of these were the great Eurypterus (Fig. 189, a) and the 

 still more gigantic Pterygotus (Fig. 189, b). The former reached a 

 length of one and a half feet or more, and the latter attained in the 

 next period (Devonian), to which this stage was a transition, a length 

 of over six feet. For crustaceans, these were truly gigantic dimen- 

 sions, and were probably never surpassed. These giants among their 

 kind seem clearly to have been aquatic forms, but whether they were 

 primarily marine or fresh- water habitants is not so obvious. They 

 are wholly extinct, and their habitat can only be inferred from their 

 associations. Some crustacean fragments that seem to belong to 

 the same subclass as the eurypterids (Merostomata) have been found 

 by Walcott in Pre-Cambrian beds, 1 but their associates are too few 

 to throw much light on this question, though they favor a marine 

 habitat. A very few eurypterids appear in the Ordovician, where 

 they are associated with marine invertebrates. In the Waterlime 

 beds they are associated with ceratiocarids and ostracodes which are 

 usually marine, and, very rarely, with certain brachiopods, which 

 1 Bull Geol. Soc, Am., Vol. X, pp. 199-224, 1899. 



