VERTEBRATES. 441 



Lateral faces moderately convex, gently compressed posteriorly, more 

 rapidly converging in front, aud ornamented with rather coarsely sculp- 

 tured stellate tubercles, which are arranged in closely approximated 

 longitudinal rows and irregular oblique order ascending from the ante- 

 rior to the posterior border, with faiut intervening sulci, and vertically 

 spaced by two to four times their diameter ; the tubercles present con- 

 siderable variation in sculpturing, their apices are obtusely acuminate 

 and directed toward the base, perceptably diminishing iu size posteriorly, 

 though not regularly, small tubercles being interspersed with the 

 larger ones in the anterior portion of the spine. Intercostal spaces 

 marked with delicate impressed stria?. Posterior face about two-thirds 

 the breadth of the spine in its thickest part, prominently keeled, faintly 

 depressed laterally and distinctly defined in the obtuse posterolateral 

 angles. Internal cavity about half the diameter of the body, similar in 

 section, posterior walls very thin, and probably opening below the dor- 

 sal line, deeply excavating the posterior face of the base. Base stroDg, 

 deeply inserted, tapering to the obtusely pointed inferior extremity, 

 dorsal line indicating a posterior inclination of about 45°. 



The only specimen of the above species which we have examined, 

 represents a nearly entire spine about two aud a half inches in length. 

 The thin walls of the posterior face, have been crushed in along the 

 lower portion of the spine, showing the extent of the internal cavity ; 

 otherwise the state of preservation of the specimen clearly exhibits its 

 generic identity and specific peculiarities. In general outline and size 

 it is very like G. bullatus of the Chester formation ; but the anterior 

 keel is less broadly rounded in front, and the tubercles are far less 

 crowded vertically, while their coarse radiating ridges offer marked 

 contrast to the smooth or faintly sculptured tubercles of the Chester 

 form. 



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

 Louis, Missouri. 



Geisacanthus btjllattjs, St. J. and W. 



PI. XVII, Fig. 3,4. 



An imperfect specimen, indicating a spine probably at least two 

 inches in length, but broken away at either extremity and otherwise 

 mutilated, so that only a small part of the specimen preserves the super- 

 ficial characters by which its generic peculiarities have been recognized, 

 affords the following specific features : Spine strong, somewhat rapidly 

 tapering towards the apex, transverse section subtriangular or conical, 

 thickness about half the anteroposterior diameter. Anterior edge 

 gently arched and traversed by a prominent, rounded and laterally 



