terminal spine is short and stout with incurved point. All the segments, including the terminal spine, carry sharply elevated longitudinal ridges.
On account of the obvious distortions of all specimens we refrain from giving measurements of the parts of the animal beyond the relative dimensions cited above.
The metastoma and genital appendages have not been observed in place.
Horizon and locality. One of the rarer forms in the Shawangunk grit at Otisville, N. Y.
Remarks. S. (Ctenopterus) cestrotus stands apart from all its allies in a number of characters that show it to be an aberrant form. The most notable of these are the frontal prolongation of the carapace, the frontal row of denticulations, the strongly tuberculate mound behind the latter and the submarginal, forward position of the eyes. It is hardly to be doubted that these characters together with the slender form of the body and the length of the legs indicate that it was an active species and not a mud groveler. The elongate outline of the carapace it has in common with S. excelsior. Both these species probably belong to the same subgenus, Ctenopterus.
Different degrees and directions of compression have produced a strikingly varying series of aspects of the carapace, some of which are here reproduced, since these specimens serve to bring out certain other features. In some we have indicated the direction of compression by pointers. In a few [pl. 50, fig. 4–6] the typical aspect is so completely changed by the preservation that without intermediate stages they would surely be taken for. representatives of different species.
There is a distinct similarity by convergence between this form and certain species of the trilobite Dalmanites that finds its most pregnant expression in the frontal row of denticulations duplicated in the subgenera Odontocephalus and Corycephalus, in the bulging frontal mound, recalling the frontal lobe of the glabella, and the large, widely separated crescent-shaped eyes. We can hardly go amiss in attributing this similarity less to an accidental coincidence in fugitive characters than to an adaptation to like conditions or similar habits.
