VERTEBRATES. 477 



which is probably a persistent feature of the form. The surfaces of the 

 spine are studded with relatively large, coarsely stellate tubercles, inter- 

 spersed with smaller ones, and more or less irregularly disposed, and 

 which are coarsely sculptured in radiating ridges, though generally 

 worn smooth, abruptly terminated above, the apices directed toward the 

 summit of the spine, which latter features are, perhaps, most apparent 

 in the upper portion, where the larger tubercles themselves present the 

 appearance of eccentrically apiculate scutes. The interspaces are irreg- 

 ularly and finely striated or striato-punctate. A specimen below the 

 medium size has a length of at least 1 iucb, transverse diameter near 

 the middle .07, greatest expansion near the tip about .10, transverse 

 basal expansion about .20 inch; lateral diameter reduced to the mini- 

 mum by the compression of the opposite sides. 



The general aspect of the spines described above bear a striking 

 resemblance to Pnigeacanthus ; but their greater length and peculiar 

 armature of the apex widely distinguish them. Whether the absence 

 of the apical processes in the above form should be regarded as evidence 

 of its generic distinctness from the spines under consideration, we may 

 not presume to have conclusively demonstrated ; but that the facts bear 

 snch interpretation seems, to us, unquestionable. 



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

 Missouri, Alton, Illinois. 



Genus OKACANTHUS, Agassiz. 



Or acanthus J ? obliqutjs, St. J. and W. 



PL XXII. Fig. 16. 



A fragment of a spine, too imperfect for satisfactory description, but 

 presenting anomalous features distinguishing it from forms previously 

 noticed in our Carboniferous collections, we have provisionally referred 

 to Oracanthus, with which in some respects it seems to be closely allied. 

 The spine appears to have been of moderate length and rapidly tapered 

 to the obtuse apex, but the posterior borders are broken away and the 

 remaining lateral surfaces bear evidence of compression, thus suggest- 

 ing the presence of a large central cavity. The anterior margin, which 

 is gently curved posteriorly, presents an obtuse angle bearing prominent 

 eccentric, obliquely trausverse decussations, which gradually diminish 

 and are finally replaced by simple tubercles on approaching the apex 

 similar to those occurring in the lateral faces ; the transverse tubercles 

 are widely spaced, narrow along the crest, abruptly descending above, 

 sometimes interrupted giving rise to a series of small tubercles, and in 

 their worn crests coarsely puuctate. The lateral faces are studded with 



