VEETEBEATES. 465 



"which are widely spaced above, but approaching the base frequently 

 bifurcate, becoming more crowded and scarcely distinguishable from 

 the intercostal ridges, which latter also bifurcates, and of which there are 

 two to four iu the upper intercostal spaces and separated by delicate 

 punctate sulci ; the costse swell at regular, though rather widely spaced 

 intervals of four or five times their diameter, into obliquely elongated 

 tuberculose prominences, the inferior bifurcations being more frequently 

 interrupted, and the anterior ridge marked by distinct conical, closely 

 arranged tubercles, similar to those in the anterior margin, but smaller; 

 the intervening anterior intercostal space is relatively wide, the bifur- 

 cated ridge below terminating iu isolated, minute tubercles. The pos- 

 terior face is abruptly truncated, the mediau line occupied by a row of 

 prominent, laterally compressed, vertically elongated, conflueut, smooth 

 denticles, whose apices are directed slightly downward, and forming an 

 interrupted and deeply notched keel ; the postero-lateral angles occu- 

 pied by a strong ridge bearing tuberculose denticulations, similar to 

 those of the posterior keel. Base strong, rapidly converging interiorly, 

 deeply inserted forward at an angle of about 50° with the dorsal line. 

 Pulp-cavity small, subovate or circular in section, and situated posterior 

 of the middle. The largest specimen known attains two and a half 

 inches in length, greatest diameter in the middle .12, thickness about 

 .05 ; pulp-cavity about .03 in greatest diameter at the same point. 



A rare form, we have had for examination only a few fragments of 

 spines, besides the beautiful and well-preserved original specimen 

 described by Messrs. Newbekby and Worthen. But so strongly 

 marked are the generic and specific characters in these spines that 

 the merest fragments, showing the superficial ornamentation and sec- 

 tion, are sufficient for their identification. 



Position and locality : Upper beds of the St. Louis limestone; St. 

 Louis, Missouri. 



GrEsrus MAKRACA^THUS, St. J. and W. 



Dorsal spine nearly straight or with a forward curvature, obtusely 

 terminated, rounded in front, truncated behind or rounded into the pos- 

 terior face, which is longitudinally ridged iu apparent continuity with 

 the lateral costa3. Lateral face and anterior margin longitudinally 

 ridged, the costse being tuberculated, those in front more or less strongly 

 developed, with their apices directed upwards, and especially in The 

 upper part where they gradually increase in size, forming strong, more 

 or less deflected hooks, transversely carimited ; intercostal spaces 

 minutely ridged and striato-puuctate. Base moderately inserted, lorin- 



— CO 



