retained its primitive character; and while the legs are of uniform length in Pterygotus, in all other eurypterids, mature and adolescent [E. remipes, pl. 2], they form a series that is longer backward.[1] It is therefore to be concluded that, even if the legs were originally of uniform length, a differentiation in length took place very early. The relatively great length of the few segments observed in Strabops indicates that the last pair of legs had attained considerable length and that the differentiation had already commenced in the Cambric. We have accordingly restored Strabops with a series of gradually lengthening slender, nearly spineless legs and would provide the prototype with a similar only more uniformly long series of legs.[2]
The chelicerae, or preoral appendages, are of identical structure in all—only excessively enlarged in Pterygotus—and therefore not available for inferences as to the prototype except that it had them as seen in the majority of the forms.
The opercular appendages have not been observed in either the larvae or in Strabops and are therefore useless for the present inquiry. Laurie has suggested that the median lobe of the genital operculum in Pterygotus shows a less degree of development than in Slimonia and Eurypterus, but
- ↑ Eusarcus makes a partial exception, inasmuch as the second pair is the longest and the following legs are successively shorter; but as the first pair of legs is the shortest, it is obvious that this reversal is a secondary modification.
- ↑ In order to get a transitional form from the eurypterids to the larval Ammocoetes, Gaskell [Origin of Vertebrates, p. 242] has "made the four endognaths small, mere tentacles in recognition of the character of these appendages in Eurypterus" [see text fig. 16]. While it is admissible to assume the possibility of such reduction of the endognaths for purposes of a hypothetical phylogeny, the fact should not be lost sight of that we have no evidence whatever of any tendency among the eurypterids toward a suppression of these appendages to mere tentacles, but on the contrary it is very clear that in all branches there is a progressive lengthening and development of the legs, with the exception of Pterygotus where 4 pairs remain primitive, but without reduction. The growth in relative size is most notable in the Stylonurus branch, but is observable in the central Eurypterus stock as late as Carbonic time. Gaskell's figure and description must then have reference to a divergent, early branch of the eurypterids, whose existence is quite unknown.
