80 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
And sometimes, when a tubercle arises in an intercostal space and does 
not fuse with its neighbors on either side, its base may become elon- 
gated, other tubercles succeed behind it, and thus a new longitudinal 
ridge is formed. Owing to the increase in width of the spine toward 
the base, it rarely happens that after a ridge is once formed it is crowded 
out. 
The slight concavity of the anterior margin, as seen in the type, 
might lead one to suppose that the spine was actually curved forward, 
but this appearance is in all probability due to distortion. The spine 
is preserved for a length of 16 cm., and has a maximum width of 
4.5 cm. Its thickness at the base where the cross-section shown in 
text-figure No. 8 is taken, was probably about 1.8 cm.,— making due 
allowance for the effects of crushing ; and the pulp cavity remains open 
for a distance of 12.5 cm. The two parallel costz extending along 
the anterior margin —it is not sharp enough to be called an edge — 
differ in no respect from the rest. 
To this species is also referred a smaller and more fragmentary speci- 
men belonging to the Wachsmuth Collection in the Museum of Com- 
parative Zodlogy, obtained thirty years ago from the Kinderhook beds 
of Burlington, Iowa. That it is a young example, and not the distal 
end of a fully grown spine, is apparent from several reasons, such as 
the position of the pulp cavity as seen in cross-section, the straight 
course of the longitudinal costz, and general delicacy of the specimen. 
As many as twelve parallel costz occupy a space only 7 mm. wide. 
The nodes are separated by about the same intervals relative to the 
size of the costa, as in the adult spine just described ; their striation, 
however, is much obscured by weathering or wear. An enlarged view 
of its ornamentation is shown in text-figure 8 B. 
Formation and Locality. — Kinderhook Limestone ; Iowa. 
Ctenacanthus lucasi, sp. nov. 
Plate 6, Fig. 1. 
The spine which is shown of slightly less than the natural size in 
Plate 6, Fig. 1, unfortunately lacks the greater part of the exserted 
portion, but so far as can be judged from the curvature, form of cross- 
section, and oblique line of insertion, it was of the same general form 
as CO. depressus and C. venustus from the same horizon. Hence its 
position may be interpreted as having been in advance of the first 
