Ctenacanthus Spines from the Keokuk Limestone. 11 



this form, both in size, shape, and 

 details of ornamentation. The 

 longitudinal costse are of round 

 cross-section, about the same dis- 

 tance apart, and are denticulated 

 or collared in the same manner in 

 both forms. In G. xiphias, how- 

 ever, they arise less frequently by 

 dichotomy, and as far as can be 

 learned from the present speci- 

 men, the series of denticles on the 

 posterior face is nearly obsolete. 

 Only the bases of these denticles 

 are preserved on the specimen, 

 but they are quite faint, and 

 appear to be limited to the distal 

 half of the spine. The exserted 

 portion is very obliquely demar- 

 cated from the base, and the pulp- 

 cavity remains open for quite a 

 distance beyond it. No evidence 

 of a keel appears on either face, 

 but along the anterior margin two 

 longitudinal costse, arising one on 

 either side, unite to form a blunt 

 ridge serving as a cutwater. The 

 inserted portion tapers more grad- 

 ually than in C. denticulatas and 

 most other species. A knowledge 

 of these characters enables us to 

 frame a more complete diagnosis 

 of this species, as follows : 



Ctenacanthus xiphias (St. John 

 and Worthen). 

 Spines attaining a length of over 

 20 cm , moderately curved, the ante- 

 rior margin more strongly convex 

 than the posterior ; gradually taper- 

 ing, and laterally very much com- 

 pressed. Base deeply embedded, 

 tapering, and obliquely marked 

 off from the exserted portion ; pulp timesenlarged. 

 cavity continued in a posterior 



channel for about half the total length of the spine. Denticles 

 oi' posterior angles uniformly spaced, more or less rudimentary. 

 Ornamentation consisting of parallel longitudinal ridges, rarely 

 bifurcating or implanted, rounded, about their own diameter 



J.W.F. 



Fig. 1. — Ctenacanthus xip7iias(St. 

 John and Worthen). A, Spine f 

 natural size. B, Ornamentation 4 



