VERTEBRATES. 475 



whose apices are directed towards the summit, and more or less regu- 

 larly arranged in closely approximated longitudinal rows and oblique 

 series in both directions, vertically spaced by once to twice their diame 

 ter, and of nearly uniform size, or diminishing in size above. The inter- 

 spaces are occupied by rather coarse, interrupted stria?, with more 

 distinct but irregular intercostal sulci. Pulp-cavity very large. 



The solitary fragment under consideration exhibits so small a section 

 of the spine as to render a perfectly satisfactory comparison with other 

 forms impossible. The character of its tuberculation bears a marked 

 resemblance to Pnigeacanthus deltoides, as also the rapidly tapering out- 

 line; but the presence of strong denticles in the concave margin, also 

 the more definite longitudinal disposition of the tubercles in the lateral 

 surfaces, distinguish it from that form, and at the same time present 

 close affinities with Gampsaeanthus, to which it is here referred. 



Position and locality : In the upper fish-bed horizon of the Keokuk 

 limestone ; Boouville, Missouri. 



Genus LE OK ACANTHUS, St. J. and W. 



Spines elongated, laterally compressed, rounded in front and behind, 

 tapering posteriorly (?), curved, base expanded before and behind, very 

 thin, apparently restiug upon or but slightly imbedded iu the integu- 

 ments, apex more or less transversely expanded and armed with strong 

 denticles. Lateral surfaces rounded into either margin, and occupied 

 by irregularly disposed stellate tubercles, those iu the upper porlion 

 have their apices above the middle, while in the lower portion of the 

 body they are more symmetrical, or subcorneal. Pulp-cavity very large, 

 central, the inclosing walls correspondingly thin, apparently thickest in 

 the convex margin. Spines of small size. 



The spines here indicated are closely allied to Pnigeacanthus, appa- 

 rently possessing the same basal expansion and very similar tubercula- 

 tion of the lateral surfaces and borders. The tubercles appear to be 

 uniformly distributed in the sides and anterior and posterior borders, 

 though they are sometimes enlarged in one or other margin, large and 

 small intermingled without distinct arrangement. But the most marked 

 distinguishing feature is the singularly distended, spinose tip, which 

 presents a striking contrast to the simple tuberculated apex of the above 

 mentioned spines. In general outline, and perhaps tuberculation, these 

 spines seem to be intimately related to Dipriacanthus, McCoy ; but their 

 relationship is no more intimate than is that of Gampsaeanthus, while 

 the description of Dipriacanthus (Brit. Pake. Foss., p. G27, PI. 3 k, fig. 

 18,) does not permit the satisfactory determination of its affinities with 

 either of the genera here indicated. 



