Eurypterus, Hughmilleria, Stylonurus and Pterygotus from the nepionic, almost microscopic, stage onward. These ontogenetic stages have been carefully described in the present work and their bearing on the much discussed relationship of the eurypterids to Limulus and the scorpions estimated.
The third fauna is, as already noted in the preface, a discovery of quite recent date made by the junior author in 1910 in the Frankfort shale (Upper Lower Siluric) along its outcrops in the counties of Schenectady and Schoharie. This material is not very well preserved and owing to its incomplete character we are for the present forced to form our conception of specific values from the carapaces alone and to unite with them such other parts of the test as are presumably referable to them. For this reason too, the generic references must be regarded as open to question and of provisional value only, there always being the presumption that when the full anatomy of these Lower Siluric creatures becomes really known they will prove to be genetically unlike the species of later date. While the morphology of these species is not yet wholly clear, the age of the fauna is a factor of chief interest, for the Lower Siluric has hitherto afforded only a few fragments known under the names Echinognathus clevelandi and Megalograptus welchi. With our present knowledge of this assemblage we are entitled to the inference that in a late stage of the Lower Siluric the eurypterids had attained a diversity and an abundance quite as great as in the Upper Siluric. We estimate this diversity in some measure on the striking differences in test sculpture presented and even though this may be an unsafe guide to either specific or generic distinctions yet these sculptures are in so large measure unlike those of better known species that they must be given full worth. These characters are fully elucidated at the proper place in the descriptive part of this book and there are among them undeniable evidences of ornament which we have come to recognize as indicative of the genera Eurypterus, Eusarcus, Hughmilleria and Pterygotus. Hence these and other outstanding terms have been adopted in the charac-
