The swimming leg [pl. 81, fig. 10] agrees with the corresponding organ of its allies in its smaller elongate paddle and long seventh segment.
Metastomas and telsons, referable to Pterygotus, have also been observed in the Otisville beds. The latter are of bilobed form and indicate Huxley & Salter's genus Erettopterus. The half of one telson, obviously too short, through anteroposterior compression shows a fringe of very minute, acute points such as have, to our knowledge, not been observed before on the telson of a Pterygotus. Smaller specimens (as that reproduced in plate 82, figure 12) are better preserved and consist of elongate bilobate bodies with a central, posteriorly tapering, raised axis. The lobes are broad and well-rounded.
While the carapace of this species strikingly resembles that of P. (Erettopterus) bilobus Huxley & Salter [see especially Woodward, 1869, pl. 10, fig. 3] in its subcircular outline and larger eyes, the telsons from Otisville are much shorter and less bilobed and can be only compared to those of P. (Erettopterus) banksii [Huxley & Salter, Monogr. pl. 12, fig. 23, 36, 37]. Both carapaces and telsons thus show closest relationship to species of the subdivision Erettopterus, a fact that argues for the union of the carapace and telsons.
Ontogeny. By far the most interesting part of this small series of specimens are the four immature individuals [pl. 82, fig. 1–4] which give us the first information on the ontogeny of a Pterygotus. Of these four two are clearly in a larval condition and represent the nepionic stage, while the others have certain characters in common which, together with their size, support the view that they are still immature and belong to the type intermediate between the nepionic and ephebic conditions.
The two nepionic specimens are characterized above all by the relatively great size of the carapace, the small size of the abdomen and small number of segments. The carapace occupied nearly one third the length of the whole body while in the mature P. macrophthalmus, the only species of which we have whole individuals, it reached only about one eighth the length of the body. The fourth figure indicates how far in
