Remarks. The outline of the carapaces is quite variable, at times being nearly quadrilateral [pl. 86, fig. 3], at others almost semicircular. Much of this diversity of form is obviously due to the compression and wrinkling of the rather flaccid carapaces in different directions, but in the best preserved specimens there is still an element of difference left after allowing for all the secondary influences that suggest the presence of more than the two species of Pterygotus here described and a distinction between the round and the squarish carapaces. This suspicion is strengthened by the evidence from the patches of integument showing patterns of ornamentation which also indicate a greater number of species of Pterygotus. We have selected the squarish specimens as typical of P. prolificus.
Disjecta membra of Pterygotus from the Frankfort shale
Besides the carapaces here used for specific diagnosis, many other fragments are referable to Pterygotus. The most characteristic of these may be briefly noted.
Plate 86, figure 19 represents a fragment of the arm of a pincer.
The distal end of a swimming leg, which in the form of the seventh and eighth segments resembles the leg of a Pterygotus more than that of any other genus, is reproduced in plate 86, figure 16. In the same group belongs plate 86, figure 20. There occur entire large swimming legs which probably belong to O. prolificus. Another very characteristic group of fragments are the female opercular appendages [pl. 87, fig. 1–3]. They resemble those of the giant P. anglicus and from their dimensions may belong to P. prolificus. Still another form of a large appendage corresponding to a type observed at Otisville, is seen in plate 87, figure 4. The most remarkable of the opercular appendages is one [pl. 86, fig. 11] which beautifully retains the characteristic broad overlapping crescent-shaped scales of the Pterygotus-Slimonia group. This resembles the male opercular appendage of Slimonia.
The metastoma reproduced in plate 86, figure 17, resembles in the anterior half, which alone is preserved, so much the metastomas of several species of Pterygotus and also corresponds in its large size so perfectly to the parts of P. prolificus that there is little doubt of its belonging with the large carapaces of that species.
