248 PALEONTOLOGY OF ILLINOIS. 



spine, which probably attained a length of at least 150 mm., and 

 though somewhat distorted by pressure, it presents in a very satis- 

 factory manner the details of the surface ornamentation of a very 

 handsome fin-defense, of which illustrations are given in the 

 plate cited. Toward the tip in the older portions of the spine the 

 lateral costae are worn nearly smooth along their crests, though re- 

 taining vestiges of the transverse tuberculations in the pectinated 

 lateral borders. Toward the base the costae show uniformly less effects 

 from wear, where the transverse ridges are beautifully preserved. 

 But throughout its entire length the dorsal ridge presents the same 

 uniform plain, smooth surface, an effect which it would seem in- 

 consistent to attribute to abrasion. The specimen also clearly shows 

 the form of the posterior denticles, as described, and which offer 

 altogether an anomalous feature compared with the downward- 

 hooked denticles occurring in the borders of the posterior face of 

 typical Ctenacanthus. 



Geological position and locality: St. Louis limestone : Alton, Illi- 

 nois. 



GENUS ASTEROPTYCHIUS, Agassiz. 



ASTEBOPTYCHIUS TBNELLUS, St. J. and W. 

 PI. XXI, FiR. 4. 



Fin spine small, gradually tapering and moderately arched along 

 the anterior edge, much compressed laterally, giving the transverse 

 section a thin wedge-shaped outline, posterior face unknown. Lat- 

 eral surfaces gently convex transversely, occupied by regular, delicate 

 rounded costse, intercostal furrows, increasing in width toward the 

 anterior edge, the posterior furrows occupied by a single striato- 

 punctate line, the middle and anterior ones showing two or more 

 such lines separated by thread-like carinae, the wide depression be- 

 side the anterior keel bearing several of these intercostal striae and 

 filiform ridges ; the anterior keel, like the lateral costae, is enveloped 

 in a smooth enamel layer. The pulp cavity occupies about half the 

 diameter of the spine, presenting an angular-ovate section, from 

 which may be inferred the keeled condition of the posterior face. 



The above species is represented by a fragment apparently be- 

 longing to the middle portion of a small delicate spine, which shows 

 the anterior keel and the lateral surface nearly to the postero-lateral 

 angles; but the posterior walls are broken away, exposing to view 



