112 



UNITED STATES NATIONAL MUSEUM. 



These spines have the usual oblique ends, so that in life they would be directed 

 outward and backward at a consideAble angle from the perpendicular (see fig. 65). 

 The lower ends of the anterior pair are rugosely roughened with weakly developed, 



H asymmetrically placed 

 longitudinal keels ; the 

 posterior pairis smooth 

 and without keel. 



The spines are unus- 

 ually flat and slender 

 with sharp edges fore 

 and aft throughout 

 their entire length. 

 The lower part of the 

 shaft is slightly con- 

 stricted antero-poste- 

 riorly so that the 

 widest portion of the 

 blade is some 200 or 

 250 mm. above the 

 basalend. Inallother 

 species the spines grad- 

 ually taper from the 

 base to the very tips 

 (see fig. 60). 



The long, slender, 

 flattened shafts with 

 sharp edges; the con- 

 striction of the shaft 

 above the bases; and 

 the uniformity in the 

 development of the 

 bases of the spine series 

 are a combination of 

 characters which will 

 at once separate the 

 tail spines of S. longi- 

 spinus from other de- 

 scribed species. The 

 principal features of 

 these spines are well 

 shown in figure 66. 



Turning now to the caudal vertebrae we find that the anterior members of the 

 series have the usual short centra and robust spines with expanded summits; 

 the median caudals are more elongated with hexagonal centra, and appear indis- 

 tinguishable from those of the type of S. stenops. 



