Page:The Eurypterida of New York Volume 1.pdf/116

 p. 198] makes a positive statement that the Carbonic  is a fresh-water form, basing his conclusion on the character of the rocks and the associated flora and fauna.

There seems hence little doubt that the eurypterids of the Carbonic finally abandoned the sea and entered the fresh water. Directly thereafter, in the Permic they became extinct, the last of the race being the  de Lima, found in the Rotliegende of Portugal associated with   and.

Summarizing these data we conclude that the eurypterids lived in the sea from Cambric to Siluric time. They had then become less sensitive to changes, positive and negative, in the salinity of the water. In fact they seem to have thrived best under conditions of life that exclude most other marine groups of animals, that is, in the marginal, more or less inclosed marine lagoons, accompanied by estuaries receiving delta-forming terrestrial drainage, with prevailing arid or subarid climate, the waters being in some places more than normally briny, in others having less than normal salinity. In other words they were euryhaline or able to live in both salt and brackish water.

Their adaptation to such conditions is paralleled today by such crustaceans as Apus and Artemia which not only thrive under rapid diminution of normal salinity but, by means of strongly protected eggs, even survive salt pan conditions which end in complete desiccation, as shown by their well known occurrence in desert lakes. The usual associates of the Siluric eurypterids are peculiar crustaceans whose nature emphasizes the reference above made. They are phyllocarids and ostracods and members of the strange family Hemiaspidae (Neolimulus, Bunodes, Hemiaspis, Pseudoniscus). This congeries of peculiar crustaceans seems to constitute a fauna especially adapted to, and therefore highly characteristic of, lagoon and estuary conditions.

Thus while the earlier eurypterids were marine and their climacteric fauna euryhaline; their later habit throughout the Devonic and Carbonic led them finally into the fresh water.