EASTMAN: CARBONIFEROUS SHARKS. 



83 



ances, which are larger than the adjacent tubercles, but not sufficiently 

 developed to be styled denticles. A prominent median ridge extends 

 along the posterior face from the opening of the pulp cavity to the 

 apex. The cross-section shown in text-figure 10 is taken at a distance 

 of 8 cm. from the tip of the inserted portion, that in 10-4 at a distance 

 of 15 cm., and that in No. lOi? at a distance of 22 cm. 



A. 



B. 



Fig. 10. 



Ctenaranthus venustus, s-p. nov. Cross-sections near the base, middle (A), and distal 

 portion (B), of the type-specimen. X x- 



About 55 longitudinal costas are to be counted along the line of 

 insertion in the larger specimen, which has a total length of 34 cm. 

 and about 40 in the original of Plate 3, Figure 2, which is preserved 

 for a length of a little over 14 cm. The type of C. depressus Newberry, 

 which is intermediate between these two examples in size, has only 

 30 continuous longitudinal costse, and is apparently less laterally 

 compressed. As representatives of the group of slender, elongated 

 spines, which are supposed to be correlated with the anterior dorsal 

 fin, the species known as C. depressus, C. venustus, and C. lucasi stand 

 in the same mutual relationships as do the stumpy and abbreviated 

 spines from the same horizon, such as C. spectabilis, G. varians, C. 

 dejlexus, C solidus, etc., those of either group being distinguished from 

 one another chiefly by ornamental details. 



Formation and Locality. — Kinderhook Limestone ; Iowa (]). 



Ctenacanthus, sp. indet. 



A very large spine, evidently of this genus, was obtained by Mr. L. A. 

 Cox from the Keokuk limestone in the vicinity of Keokuk, Iowa, and 

 is now preserved in the United States National Museum (Cat. No. 

 3480). It is much abraded, and only the exserted portion remains. 



