THE EURYPTERIDA OF NEW YORK 21 
Eurypterus, Hughmilleria, Stylonurus and Pterygotus from the neptonic, 
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 generically 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 Echinogna- 
thus 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- 
