EASTMAN: CARBONIFEROUS SHARKS. | 77 
tion, and whose total length was probably about 15 cm. It is to be 
hoped that eventually a correlation may be established between Edestus 
and some of the huge dermal defences, such as Oracanthus, for instance, 
which accompany it in the Carboniferous. 
II. ON SPINES OF CTENACANTHUS FROM THE 
MISSISSIPPIAN SERIES. 
It is customary to recognize Ctenacanthus as a distinct genus, for 
although the spines are indistinguishable from those of Hybodus, they 
are not associated with Hybodus-like teeth in the Devonian and Car- 
boniferous, none such having been found in rocks older than the Meso- 
zoic. Newberry has brought forward some evidence to show that 
Orodus possessed spines of this nature, and this association is entirely 
consonant with the fact that Orodus and Campodus are’ Paleozoic fore- 
runners of Hybodus. In view, however, of the almost universal occur- 
rence of the spines of Ctenacanthus in a detached condition, it is proper 
to retain this as a provisional genus of Cestraciontide until such time 
as its relations have been definitely established. 
Of primary importance in the distinction of species is the general con- 
formation of the spine, especially its curvature, form of cross-section and 
length of inserted portion. Next in order are to be considered the 
number, shape, and direction of the longitudinal cost, with the orna- 
mentation of the same; and still further distinctive characters are to be 
found in the nature of the posterior face and anterior margin, or “ cut- 
water.”” Sometimes weight has been placed on the above characters in 
reverse order from that indicated, and this has led to the establishment 
of doubtful species, or even genera of Ctenacanthus-like spines, such 
as Anaclitacanthus semicostatus, Hunemacanthus costatus, the types of 
Acondylacanthus xiphias, Ctenacanthus limaformis, ete. 
Species of Ctenacanthus are especially numerous in the Kinderhook 
limestone of Iowa in the vicinity of Burlington, and elsewhere along the 
Mississippi River ; and these may be divided into two general groups. 
The series formed by C. varians, C. spectabilis, C. deflexus, C. solidus, C. 
clarki, and C. brevis is characterized by abbreviate, stumpy proportions, 
by a similar pattern of ornamentation, and by having been inserted 
very obliquely in the integument. Spines of this nature are to be re- 
garded as having pertained to the second (posterior) dorsal fin. They 
contrast strongly with the group of slender, elongated and tapering 
