The telson exhibits two very distinct lines of development. It is lanceolate in most genera, notably Eurypterus, Dolichopterus, Eusarcus and Hughmilleria, becoming in extreme cases styliform, as in the Stylonuri and some of the earlier Eurypteri and the later Anthraconectes; then again it is broad and bilobed in the subgenus Erettopterus. Laurie leaves the question open as to which style of telson is the more primitive, the geological succession having given him no clue. He suggests, however, that as the pointed telson is characteristic of the earlier trilobites, it also is of the eurypterids. Our investigation supports this suggestion. Strabops possesses a short and blunt pointed telson which fully corresponds with the telson of the Siluric larvae, indicating that this is the primitive form of that organ. The primitive Hughmilleria possesses a similar telson, though it already exhibits a tendency to a broadening and flattening of the proximal portion. Slimonia has this tendency still more developed, so that the telson appears as lanceolate with winglike lateral extensions of the anterior half. A reduction of the posterior spine may then be conceived to produce the telson of Pterygotus, and a further suppression of the axial lanceolate portion would lead to the bilobed telson of Erettopterus. The telson of Hughmilleria, Slimonia, Pterygotus and Erettopterus thus seems to present a continuous series of developmental stages, and the pointed or bluntly lanceolate-triangular form to be that of the prototype of the eurypterids.[1]
Genealogy. In the preceding reconstruction of the prototype we have indicated our views of the genetic relationships of some of the genera,
- ↑ The remarkable observations of Walcott on the trilobites of the Mesonacidae [Smithsonian Miscell. Coll. 1910, v. 53, no. 6] show that the telson of Olenellus is not a true pygidium but originates from a median spine of an earlier segment by the suppression of posterior segments which are still present in the genera Mesonaeis and Paedeumias. The fact that the telson of Olenellus resembbs that of Limulus has suggested to Walcott the view [p. 246] that while this resemblance does not necessarily indicate that Olenellus was the ancestor of Limulus, "its origin does indicate the manner in which the telson of Limulus may have originated."
The possibility of such origin for the telson of Limulus and the eurypterids is
