126 | NEW YORK STATE MUSEUM 
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.' It is therefore to be 
concluded that, even if the legs were originally of uniform length, a differ- 
entiation 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.’ 
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 Slmonia and Eurypterus, but 
1 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. oe 
2In order to get a transitional form from the eurypterids to the larval Ammo- 
coetes, 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 sup- 
pression 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 descrip- 
tion must then have reference to a divergent, early branch of the eurypterids, whose 
' existence is quite unknown. 
