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 A at a distance 
of 15 cm., and that in No. 10B at a distance of 22 cm. 
Ade 
Fie. 10. 
Ctenacanthus venustus, Sp. NOV. Cross-sections near the base, middle (A), and distal 
portion (B), of the type-specimen. X 4, 
About 55 longitudinal coste 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 14cm. The type of C. depressus Newberry, 
which is intermediate between these two examples in size, has only 
30 continuous longitudinal coste, 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. lucast stand 
*n the same mutual relationships as do the stumpy and abbreviated 
spines from the same horizon, such as C. spectabilrs, QO. varians, C. 
deflexus, C’. solidus, etc., those of either group being distinguished from 
one another chiefly by ornamental details. 
Formation and Locality. — Kinderhook Limestone ; Iowa (1). 
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. 
