A comparison of the structure here described with that of Pterygotus shows that the epistomas of both genera agree in general form. That of Hughmilleria is, however, the longer and more slender, an adaptation to the more elongate form of the head shield in that genus, and there are in Pterygotus no antelateral shields separated by sutures from the marginal shield.
The epistoma exists, so far as we now know, in no other genera than Hughmilleria and Pterygotus; that is, in the two genera which are characterized by a very strong development of the chelicerae, and this fact indicates that the plate itself has resulted from strains originating from these strong and active arms.
NOTE ON PTERYGOTUS COBBI var. JUVENIS nov.
See text figures 119-21
In the description of P. cobbi, in the body of this memoir, we had only two free rami of the pincers at our disposal, all that was known of the species. A large collection of eurypterid material since obtained by Mr Hartnagel in Herkimer county, contains two small specimens of Pterygotus on the same slab, one of which possesses the characteristic pincer of P. cobbi with some slight differences, of such nature as to be quite probably ascribable to an earlier growth stage of the species. We therefore refer to these specimens as P. cobbi var. juvenis.
These two specimens differ in a number of characters from both the common P. buffaloensis of the Bertie waterlime at Buffalo and the P. macrophthalmus of Herkimer county. 
Fig. 119 Pterygotus cobbi var. juvenis nov. Pincers. × 3 The most important of these differences rests in the pincers which are rounded in front as in P. cobbi and the free ramus of which shows the denticles of the type of that species but is stouter and distinctly higher in the middle. The carapace is more evenly rounded anteriorly than in the other two species here cited and especially longer (length to width as 4 : 5); the compound eyes are relatively larger while the abdomen appears slightly broader and stouter (the preabdomen is 26 mm long and 19 mm wide);
