Page:The Habitat of the Eurypterida.djvu/225

Rh coming from rivers of different continents, but there may be single cases of a family, a genus, or even a species which occurs in sediments from one land mass, which is nearly related to or identical with one in sediments coming from another land mass. In such a case, to determine true relationships one must compare the whole of each fauna, species by species, and must in addition study the ancestors of each fauna and of each species in the preceding periods wherever possible.

. Let us turn now to the placing of the various pre-Siluric eurypterids. Strabops thacheri from the Cambric is too primitive and morphologically undifferentiated to be looked upon as more than an ancestral form approaching the prototype and from which several branches of the eurypterid tree diverged. The first prolific eurypterid fauna in North America, the first to offer sufficient material and a large enough representation in genera and species to make it possible to state what are the general affinities of the fauna as a whole, is the newly discovered one in the Normanskill shales at Catskill, New York, which has so far been found to contain six species, included in five genera, but undoubtedly many more will be discovered as the material is worked over. On account of the fragmentary nature of the abdomina found, and because the carapaces are usually dissociated from the rest of the body, generic determinations have been provisional and comparisons with related species difficult. Yet the fauna shows a pronounced and altogether surprising similarity to that of the Schenectady beds (Trenton) despite the difference in age. In the case of Pterygotus? (Eusarcus) nasutus, Clarke and Ruedemann "have been unable to distinguish the Schenectady and Normanskill types," (39, 412); and have referred a number of carapaces from the Normanskill beds to P. nasutus, a species described originally from material from the Schenectady shales. Eusarcus linguatus from the Normanskill is very similar to Pterygotus? (Eusarcus) nasutus. Eurypterus chadwicki, Dolichopterus breviceps, and Stylonurus modestus are not well enough represented for relational comparisons to be made, so far as species are concerned. The finding of several Stylonurus carapaces, attached