476 PALEONTOLOGY OF ILLINOIS. 



Leoeacanthtjs tjngtticultjs, St. J. and W. 



PI. XXH Fig. 10, U. 



Oar collections contain numerous fragmentary remains of a peculiar 

 form, the exact affinities of which it is difficult positively to determine, 

 but which seems to be closely allied to Pnigeacanthus (Oracanthus jmigeus, 

 N. and W.) The specimens in no instance, save one, show the entire 

 length of the shaft ; they are about equally abundant, portions of the 

 inferior body and short sections of the curiously spinose tip. But one 

 specimen, found at St. Louis, shows the connection of the almost uni- 

 versally dissevered parts, though it is not improbable some of the short 

 specimens may prove to be immature spines. It is quite conjectural 

 whether the direction of curvature was forward or backward. If, as is 

 inferred, Pnigeacanthus was curved in the latter direction, assuming 

 from the character of the thin, transversely expanded inferior portion, 

 which presents no appreciable differences distinguishing it from the 

 above genus, then these spines are probably similarly arched posteriorly. 

 The body walls are exceedingly thin and of nearly uniform thickness, 

 perhaps slightly strengthened in the margins ; in consequence of their 

 frail build they are almost always crushed, the inner surfaces of the 

 walls more or less closely approximating. But in the collection of Mr. 

 Van Hoene, a fragment of a rather large individual exhibits the sub- 

 lenticular transverse section quite satisfactorily, showing the large 

 internal cavity and compressed or gently arched lateral surfaces, which 

 are rounded into the broad concave (posterior ?) margin, and from which 

 they gradually converge to the narrower and more sharply rounded 

 convex side. The curvature, viewed from the side, is considerable, the 

 basal portion rather suddenly expanding both in front and behind, but 

 none of the specimens retain any indication of an inserted extremity 

 ou the contrary, tbe inferior borders become exceedingly thin, and, as 

 mentioned by Messrs. Newbeeet and Woethen in connection with 

 Pnigeacanthus (0. pnigeus), the structure is that of semi-osseous carti- 

 lage, the osseous particles of which, in the process of growth, becomes 

 firmly impacted in the body of the spine. The middle portion of the 

 shaft to near the extremity is of nearly uniform dimensions, or very 

 gradually tapering to the suddenly and irregularly expanded apical 

 extremity, which appears to be thrown slightly forward and terminated 

 with several strong, talon-like, smooth processes or denticles, which 

 spring from the anterior (?) margin and lateral surfaces, more or less 

 compressed laterally, rounded and thickest along the convex side, and 

 curved forward. In many examples occurs a sudden tuberculose expan- 

 sion in the posterior (?) margin of the spine just beneath the tip, and 



