EASTMAN: CARBONIFEROUS SHARKS. 79 
fairly regular intervals of sculptured eminences or nodes (text-Fig. 8 A), 
a very unusual character in this genus, and in allusion to which the spe- 
cific title is bestowed. As regards ornamentation, a certain resemblance 
will be observed to Oracanthus and the spines described by St. John and 
Worthen as Glymmatacanthus rudis, and Batacanthus baculiformis. 
The longitudinal costz of Ctenacanthus are commonly described as 
being “interrupted” by transverse ridges or swellings, implying that 
the latter are of subordinate importance, and that the truly essential 
structures are the longitudinal ridges. A study of the mode in which 
the ornamentation originates in the present specimen is sufficient to 
A. B. 
Fie. 8. 
Ctenacanthus longinodosus sp. nov. Cross-section of spine. X 1. A, Portion of 
surface ornamentation of type-specimen. X#. B, Same of a young individual, 
from near Burlington, lowa.  X ¢. 
convince one that this is not the correct interpretation. For, on direct- 
ing our attention to the youngest part of the spine, that is to say, to 
the area along the line of insertion and along the border of the open 
pulp cavity, we find that growth of the cost proceeds in the following 
manner: Small, irregular tubercles of vasodentine are deposited at 
intervals along the line of insertion, and although their summits are 
smooth at first, they soon become angulated and striated. As the spine 
protrudes more and more from the integument, these tubercles become 
widened somewhat, and at the same time their bases are elongated in 
a longitudinal direction. Should two of the newly formed tubercles 
become sufficiently approximated, either their summits or bases coalesce. 
