YEETEBRATES. 407 



and apparently smooth. The costse bear similar but comparatively 

 minute tubercles, occurring quite regularly at intervals equal to four or 

 five times their diameter, diminishing in size posteriorly, where they are 

 also less transverse. Intercostal spaces form narrow, punctate sulci in 

 the posterior region, with one or two slightly wider ones in the middle, 

 which are traversed by a thread-like liue. Posterior face flattened, 

 rounding into the posterolateral angles, and apparently longitudinally 

 ridged with about five delicate, faintly tuberculated costse, very like 

 and iu continuation of the lateral costre. Base comparatively thin, 

 sharply carinate in front, laterally expanded, the posterior margins 

 somewhat rapidly diverging from above, forming an obtuse angle, from 

 which the inferior borders are broadly rounded to the extremity, pre- 

 senting from the front concave surfaces gently descending and laterally 

 expanding from the anterior angle, in outline spatulate. Pulp-cavity 

 relatively large, transverse section more obtuse in front and equal to 

 about half the diameter of the body, posterior walls about half the 

 thickness of those in front, and opening at or just below the dorsal line 

 iu the posterior side, forming a rounded, broad excavation in the inner 

 face of the base. A mature specimen measures little more than two 

 inches in length, of which the base constitutes about one-fourth. 



The form described is abundantly represented in our collections by 

 more or less perfect specimens, exhibiting various stages of growth, the 

 study of which has furnished a very clear idea of the persistent char- 

 acters, as well as those attributable to age. In young specimens the 

 tuberculatiou is generally more conspicuous, and the outline quite erect. 

 Considerable variation in the relative extent of the series of large tuber- 

 cles flanking the anterior margin on either side is noticed iu individuals 

 of different sizes, and therefore is apparently of merely individual 

 importance. But in the majority of specimens these large tubercles do 

 not descend below the middle of the spine, while in a few individuals 

 they extend even to the base. It is rare to find a specimen which 

 exhibits the posterior face, and our knowledge of the character of its 

 surface has been derived from carefully developing perfect specimens 

 by cutting away the matrix. By this means the costate condition of 

 the entire external surface of the spines has been conclusively demon- 

 strated, and which offers a marked distinguishing peculiarity as com- 

 pared with the serrated keel of Amacanthus gibbosus, with which the 

 present species was originally generically associated. 



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

 Illinois, and St. Louis, Missouri. 



