EASTMAN: CARBONIFEKOUS SHARKS. 



91 



figure 13) than C. spectabilis, and the sharp marginal ridge of that 

 species is here replaced by a row of tubercles somewhat coarser or more 

 elongated than the rest. This latter condition approximates that ob- 

 served in C. coxianus from the Keokuk limestone. 



In C. mayi about a dozen longitudinal costse are to be counted along 

 the line of insertion ; in C. coxianus as many as 18 or 20 ; in C solidus 

 upwards of 30 ; in C. spectabilis about 50 ; and in C. varians 80 or 

 more. Amongst these only C. varians agrees with the species under 

 discussion in having sculptured or striated tubercles, and 

 this character is also common to G. hrevis from tlie Lower 

 Carboniferous of Ireland, and to G. longinodosus, as de- 

 scribed above. It is thus apparent that the number and 

 direction of the costte are of prime importance in the 

 distinction of species. 



The spine shown in Plate 7, Figure 3, is preserved for 

 a lenirth of 9.5 cm. and has a maximum width of 3.5 cm. 

 The type-specimen belonging to the United States Na- 

 tional Museum has the costae more perfectly preserved than 

 either of the co-types. Its cross-section, shown in the 

 accompanying text-figure 13, is well displayed by the frac- 

 ture across the base, but the thickness at this point is 

 exceeded by both of the co-types. None of these spines 

 retain traces of posterior denticles, although it is very 

 probable that such were formerly present. 



Formation and Locality. — Kinderhook ; Iowa and 

 Illinois. 



Fig. 13. 



Ctenacanthus 

 solidus, sp. 

 nov. Cross- 

 section of 

 type - speci- 

 men in its 

 basal por- 

 tion. X {■ 



LIST OF SPECIES OF CTENACANTHUS OCCURRING IN THE 



MISSISSIPPIAN SERIES. 



