124 NEW YORK STATE MUSEUM 
V 
PHYLOGENY 
Since the date of Laurie’s commentary on the relations of the eurypterids 
among themselves [1893, p. 520], two important lines of evidence have 
been obtained bearing on the genetic relationships of the genera. One 
is afforded by the growth stages of the principal genera, collected at Otis-. 
ville, the other by Strabops of the Cambric. In the preceding chapter 
we have analyzed the ontogenetic characters and have fixed upon a small 
number of these that may, with a fair degree of certainty, be considered as 
recapitulations from eurypterid ancestry. As these characters are actually 
expressed in Strabops, it seems safe to use the two sets of facts for a con- 
ception of the prototype of the eurypterids, and to proceed from this 
prototype in tracing genetic relationships. Aside from our knowledge of 
the one Cambric genus, the geologic succession of the genera is of little 
aid in the problems before us, since nearly all genera seem to be present in 
the Lower Siluric. 
In the preceding chapter we have recognized as palingenetic char- 
acters of the larval eurypterids the approximation of the compound eyes 
to the margin, the prominence of the ocelli and their tumescences, the 
lack of differentiation of the abdominal segments and the smaller size of 
the telson. All of these characters, save the prominence of the ocelli, are 
typically shown in Strabops thacheri, or the Strabops stage. 
While the characteristics of this stage will be the basis of our recon- 
struction of the prototype, others may be safely inferred from a comparison 
of Strabops with the general characters of the Siluric eurypterids and from 
theories of the development of the arthropods in general. 
In Strabops the carapace [pl. 1] is short, anda little less wide. 
than the greatest width of the body. It is so in most of the later 
eurypterids, notably in Eurypterus, Hughmiulleria and Pterygotus. The 
eurypterid carapace has been formed by the consolidation of six segments, 
anda priori it should not be longer than six of them. As a matter of fact 
it approximately corresponds, as a rule, to the five first tergites in length, 
- suggesting a condensation. This it does also in Strabops and since the 
segments in that form are all remarkably short, the carapace is much 
