208 PALEOZOIC FISHES OF NORTH AMERICA. 
the facies are so diverse. We should therefore suggest to those who, hereafter in pos- 
session of more ample material, may be better able to adjust the classification of these 
fossil fishes, the question whether the long, slender, and flattened species of Clenacan- 
thus, cited above, should not be united with Leptacanthus junceus, L. Jenkinsoni, and the 
present species in a generic group, distinet from both Ctenacanthus and Leptacanthus. 
Messrs. St. John and Worthen,’ acting upon the above suggestion, 
though without reference to it, make this species and two others described 
by themselves—one from the Kinderhook of Burlington, Iowa, and the 
other from the Keokuk of Warsaw, I]]—the types of a new genus, to which 
they give the name of Acondylacanthus. 
Up to the present time we remain in ignorance of the teeth that were 
associated with these spines. They will probably sometime be found to be 
such as are known by the names of Cladodus or Orodus ; both of which have 
been seen associated with the spines of Ctenacanthus. The spines of Acondy- 
lacanthus have a general resemblance to those of Ctenacanthus, but are more 
slender than most species of that genus, and may be distinguished from them 
by the absence of all tuberculation on the parallel and relatively uniform 
costse of the enameled surface. The base is also shorter than in any spines 
of Ctenacanthus which have come under my observation, though in this char- 
acter they resemble some spines of Asteroptychius. 
So far as yet known the spines of Acondylacanthus occidentalis are con- 
fined to the Saint Louis beds of the Mountain limestone. 
ANTLIODUS ARCUATUS, Nn. sp. 
Plate XIX, Figs. 3. 4. 
Teeth about one inch in breadth by eight lines in height; outline 
elliptical ; posterior face polished, gently concave laterally, more strongly 
vertically ; upper margin subacute, lower margin bordered by about five 
relatively broad and strong enamel folds, which cover the lower third of this 
surface; anterior face equally divided between the crown and surface of 
attachment; crown portion strongly arched and highly polished, its inferior 
margin somewhat bow-shaped, and traversed by a single deep and broad 
furrow; adherent surface arched laterally, flattened vertically; root low, 
1 Geol. Survey Illinois, vol. 6, p. 432. 
