VERTEBRATES. 479 



vex, that above more abrupt and concave but presenting considerable 

 variation in form and size in various parts of the body, two or more 

 coalescing at the point of bifurcation of the costre, forming laterally 

 elongated prominences traversed by an interrupted transvere crest, and 

 one or more eccentric rows in either margin consisting of strong, oblique, 

 irregular tubercles, in size nearly as large again as those occupying the 

 lateral faces. Base unknown. Internal cavity large, nearly central, 

 the lateral walls less than half the strength of those in front and behind. 

 Transverse or greatest diameter near the middle of the spine 1 inch, 

 lateral diameter across the postero-lateral angles .32, lateral diameter in 

 front across the anterior limit of the cavity .25; antero-posterior diam- 

 eter of the internal cavity .70 inch. 



Notwithstanding the imperfect condition of the specimens referred to 

 above, a careful reference to the description and comparison with the 

 illustrations given by Professor Agassiz of Oracanthus Milleri (Pois. 

 Foss. 3, p. 13, Tab. 3, fig. 1-4,) of the Carboniferous limestonoof England, 

 leaves scarcely a doubt as to the generic identity of these fossils with 

 Oracanthus as the genus is defined by the typical representative 0. 

 Milleri, Ag. Specifically the present form is distinguished from that 

 just named by its relatively more numerous and less obliquely disposed 

 tubercles, which in shape are very like in both forms, though they 

 appear to be more isolated laterally and, perhaps, vertically than is the 

 case in 0. Milleri. 



Dr. Liedy has described a spine reported from the "Missouri Terri- 

 tory," and probably belonging to the Lower Carboniferous, under the 

 name Oracanthus retusttts (Jour. Acad. Nat. Sci., Phila., Vol. II, [second 

 series,] p. 101, PI. 16, fig. 1, 2, 3,) which nndoubtedly is closely allied to 

 the above described form. It is chiefly distinguishable by the less 

 robust and more numerous tuberculation, features with regard to the 

 variation of which we are not sufficiently familiar to decide their im- 

 port. 



In regard to their relationship with Pnigeacanthus deltoides, we have 

 little hesitation in recognizing their generic distinctness, the latter 

 having the appearance of, and probably more intimately homologous 

 with dermal scutes than with true flu or defensive spines — affinities no 

 less distinct than those which distinguish the teeth from the spines, and 

 though they are differences of degree, nevertheless they are always 

 strongly marked and unmistakable, by which the thin solid elements of 

 the integument are distinguishable one from the other. 



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

 111., St. Louis, Mo., Pella, la, 



