I30 NEW YORK STATE MUSEUM 
approximation to it in outline. The close relationship of Hughmilleria to 
Pterygotus is further evinced by the small size of the swimming legs and 
the form of the metastoma. The endognathites of the two differ, those of 
Hughmilleria and of the Eurypteridae being spinous, stout and increasing 
in length backward. The endognathites of Slimonia, except the antenni- 
form first pair, differ similarly from those of Pterygotus, and both Hugh- 
milleria and Slimonia lack the immense development of the chelicerae of 
Pterygotus, and we believe that the endognathites of these two genera have 
assumed these characters in order to perform a part of the functions of 
the chelicerae of Pterygotus and so served as organs of offense and defense, 
while those of Pterygotus remained undeveloped, being overshadowed by 
the great chelicerae. Asacorollary wemay assume that the earlier Hugh- 
milleriae approached Pterygotus in the character of their endognathites. 
The genetic relationship between the three genera of the Pterygotidae 
we conceive as shown in the genealogical tree. An unknown series of 
forms branched off from the prototype in Cambric time and led in late 
Lower Siluric time to Hughmilleria. From this was developed on one 
side Pterygotus proper which again produced the subgenus Erettopterus 
with bilobed telson, and on the other side Slimonia, which retained the 
small chelicerae but specialized in developing the second pair of appendages 
for a tactile function, in the greater development of the opercular or 
genital appendage, the peculiar exaggerated spinosity of the other endo- 
gnathites, the lobed and lanceolate telson and other features. Slimonia, 
represented by but a single species, has all the features of a local and 
aberrant type. | 
The Eurypteridae are represented by the far larger stock. Strabops ° 
points the course the development of that stock took from the prototype 
through the Lower Siluric. From the latter era we have only the endo- 
gnathites on which the genera Echinognathus and Megalograptus are based. 
These clearly represent another aberrant branch which, judging from the 
character of the multispinous endognathite, ends here, is independent or 
may lead to Stylonurus (Ctenopterus). 
