THE EURYPTERIDA OF NEW YORK 291 
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 tubercu- 
late 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. Ina 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. 
