128 NEW YORK STATE MUSEUM 
The telson exhibits two very distinct lines of development. It is lan- 
ceolate in most genera, notably Eurypterus, Dolichopterus, Eusarcus and 
Hughmulleria, 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 geologi-. 
cal 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 con- 
ceived to produce the telson of Pterygotus, and a further suppression of 
the axial lanceolate portion would lead to the bilobed telson of Erettop- 
terus. The telson of Hughmilleria, Shmonia, Pterygotus and Erettop- 
terus 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.' 
Genealogy. In the preceding reconstruction of the prototype we have 
indicated our views of the genetic relationships of some of the genera, 
1 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 Mesonacis and 
Paedeumias. The fact that the telson of Olenellus resembles that of Limulus has sug- 
gested 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 
