4S0 PALAEONTOLOGY OF ILLINOIS. 



Genus P¥IGEA0A1S t THUS, St. J. and W. 



Spines comparatively short, conical, laterally compressed, base broadly 

 expanded before and behind, without insertion, rapidly tapering to the 

 obtuse apex, which is directed posteriorly, transverse section elliptical, 

 rounded into the slightly sigmoidally curved anterior border and concave 

 posterior margin. Pu'p-cavity very large, extending nearly to the 

 tip; lateral walls very thin, slightly thickened in the margins. External 

 surface occupied by irregularly disposed, radiatingly sculptured tuber- 

 cles, sometimes arranged in obscure or interrupted longitudiual and 

 diagonal order. 



The affinities of the spines embraced under the above generic term 

 would appear to be more intimate with Oracanthus than with the spines 

 which we have described under Lecracanthus, differing from the former 

 in the irregular disposition of the tubercles and less solid build, and 

 from the latter by their shorter proportions and absence of spinose 

 tubercles in the apical extremity. 



The Oracanthus pnigeus, N. and W., of the Keokuk limestone is 

 regarded as the typical example. In connection with the original 

 description of the latter form, Dr. Newberry refers to a Devonian 

 species, which he has recognized under the name 0. abbreviatus, and 

 which he regards as closely allied to the above species. 



Pnigeacanthtts deltoides, St. J. and W. 



Oracanthus pnigev-s, Xewbeeky and WOETHEX, 1866, HI. Rep., Vol. n, p. 117, PI. XIT, fig- 3. 



The present form has a triangular outline, the anterior border nearly 

 straight or with a slight sigmoidal curvature, the opposite side gently 

 concave, and both rapidly converging to the bluntly pointed apex. In 

 the present condition of the original and thus far only authentic exam- 

 ple, the opposite lateral walls are pressed close together, the breaking 

 away of a portion of one side revealing the great extent of the internal 

 cavity and the comparatively thin crust of the inclosing walls. The 

 tuberculation consists of relatively strong, eccentrically conical, coarsely 

 sculptured, stellate tubercles, those in the lower portion of the spine 

 being more symmetrical than those towards the apex, which latter are 

 projected upward forming scute-like bosses, with a gentle inferior slope 

 and abrupt concave superior face ; the tubercles are arranged in obscure 

 interrupted longitudinal series, large and small intermingled, but the 

 largest occurring in the front half of the spine ; the interspaces are occu- 

 pied by coarse, interrupted longitudinal striae. The specimen exhibits 



