EASTMAN: CARBONIFEROUS SHARKS. // 



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. 



11. OX SPINES OF CTENACANTHUS FEOM THE 

 MISSISSIPPIAX 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- 

 bonifei'ous, 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 Palaeozoic 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 Cestraciontidae until such time 

 as its relations have been definitely established. 



Of primary importance in the distinctiou 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 ai-e 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 beea 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 se mi co status, Eunemacanthus costatus, the types of 

 Acondylacanthus xiphias, Ctenacanthus Umaformis, etc. 



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. spectnhilis, C. dejlexus, C. solidus, O. 

 darkly 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 



