254 THE CHICAGO ACADEMY OF SCIENCES. 



Free cheeks not known. Entire surface of the glabella and pro- 

 tuberant portions of the fixed cheeks finely tuberculate. 



Pygidium, aside, from its spines, more than twice as wide 

 as long, with eight marginal spines directed posteriorly, the 

 bases of the spines joined by the thickened marginal border 

 of the pygidium. The outer spine on each side is small and 

 short, the second ones are much larger and longer than any 

 of the others, between these two large spines are four small 

 ones along the posterior margin. The axis is prominent in front 

 and depressed posteriorly where it is confluent with the thick- 

 ened marginal border of the pygidium, it is marked by two 

 strong annulations, the anterior one of which is continued across 

 the limb as an elevated, rounded ridge which curves backward 

 on each side and becomes continuous with the two larger mar- 

 ginal spines. Each pleural lobe is depressed between the axis 

 and the thickened marginal border, the depressed regions on 

 each side being divided into two sub-triangular areas by the 

 elevated ridge which joins the anterior annulation of the axis 

 with the larger marginal spines. Surface of the pygidium 

 smooth. 



The approximate dimensions of an imperfect cranidium 

 are: length 4.5 mm., width along the posterior margin 10 mm., 

 greatest width in front of the eyes 5 mm., width of glabella 4 

 mm. The dimensions of a pygidium are: length, exclusive of 

 the spines, 3 mm., width 8 mm., width of axis at anterior margin 

 2 mm. , approximate length of larger spines 3 or 4 mm. 



Remarks. The head and pygidium here associated under 

 the name Odontopleura illinoiensis have not been observed in 

 actual position in such a manner a*s to show certainly that 

 they are parts of the same species, but they are associated in 

 same bed, and being the only acidaspids there present they are 

 considered as belonging together. The genus Odontopleura is 

 represented in American Silurian faunas by only two species 

 aside from the one here described. Both of these species are 

 imperfectly known, but 0. illinoiensis differs so markedly from 

 either of them that no comparisons are necessary. 



Locality. Near Joliet. 



Genus 13. CERATOCEPHALA Warder, 1838. 

 In the characters of the glabella and cheeks the members 

 of this genus closely resemble Addas pis and Odontopleura, but 

 the occipital segment bears two divergent spines which are 

 separate at their bases. 



