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 Otisville, 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 conception 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 characters 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 reconstruction 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, and a little less wide than the greatest width of the body. It is so in most of the later eurypterids, notably in Eurypterus, Hughmilleria and Pterygotus. The eurypterid carapace has been formed by the consolidation of six segments, and a 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
