472 PALAEONTOLOGY OF ILLINOIS. 



size, or diminishing inferiorly, sometimes with minute downward-hooked 

 tubercles in the interspaces ; apex armed with one or more strong denti- 

 cles similar to those occurring in the concave border. Base transversely 

 expanded and more or less laterally dilated, apparently not inserted or 

 but slightly buried in the integuments. Pulp-cavity large, nearly 

 central, walls thin below, more dense near the apex, and apparently 

 thickest in the convex margin. Spines of small size. 



The above general description applies to two or three forms occurring 

 in the St. Louis and Keokuk limestone, and which have close affinities 

 with the spines indicated under the designation Leer acanthus. { Their 

 main distinctive features are the uniform denticnlation of the concave 

 margin and the apparent greater regularity in the longitudinal arrange- 

 ment of the tubercles in the lateral faces. The character of the base is 

 not well shown in the specimens before us, but it apparently intimately 

 resembles that of the spines just referred to, and was probably lightly 

 imbedded. In the latter particular, and the posterior curvature, these 

 spines differ widely from Batacanthus, with which they have certain 

 resemblance of general outline. The form of the larger tubercles, such 

 as occur m the convex border of some of the species, bears a striking 

 resemblance to the large anterior tubercles in Dipriacanthus, McCoy, 

 and the resemblance is further carried out in the peculiarly expanded 

 base. But Prof. McCoy does not mention whether the form described 

 by him was a hollow spine, though he compares it with Oracanthus, but 

 particular stress is laid ou its resemblance to the comparatively solid, 

 articulated spines of some of the Silurokls ; while those under consider- 

 ation undoubtedly belong to Selachians, the character of the basal 

 attachment strongly suggesting relationship with the Bays or Myliodonts. 



Gampsacajmthtjs ttptjs, St. J. and "W. 



PI. XXn. Fig. 12. 



Spines of small size, considerably curved, and gradually tapering, 

 the basal portion very thin and more or less expanded before and 

 behind, the apex apparently bluntly terminated and possibly armed 

 with three or four flattened processes, that behind (?) largest and in all 

 respects like the posterior denticles, that in front smallest, with an 

 intermediate one, possibly two rising from either lateral face immedi- 

 ately beneath the crest. The body of the spines is almost invariably 

 crushed flat, but a fragment of apparently the same form shows an oval 

 transverse section, a very large and nearly central pulp-cavity extend- 

 ing to the tip and thin lateral walls, thickened in the margins, that of 

 the convex margin apparently strongest; this would seem to indicate 

 the posterior curvature of the spine. The lateral faces are marked with 



