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 

 Wortheu as Glymmatacanthus rudis^ and Batacanthus baculiformis. 



The longitudinal costte of Cteuacanthus 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 studv of the mode in which 

 the ornamentation originates in the present specimen is sufficient to 



Fig. 8. 



Cteuacanthus longinodosiis sp. nov. Cross-section of spine. X \- A, Portion of 

 surface ornamentation of type-specimen. X*. ^, Same of a young individual, 

 from near Burlington, Iowa. X f- 



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 costse 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 augulated 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. 



