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 antenniform first pair, differ similarly from those of Pterygotus, and both Hughmilleria 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. As a corollary we may assume that the earlier Hughmilleriae 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 endognathites, 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 endognathites 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).
