438 PALAEONTOLOGY OF ILLINOIS. 



absence of the tubercles in the anterior channels, which latter may be 

 due to accident. But, on the whole, it is a well-marked form, any frag- 

 ment showing a complete transverse section and the superficial orna- 

 mentation being sufficient for its identification. 



Compared with A. triangularis, N. and W., of the Upper Burlington 

 limestone, its more slender proportions are sufficient to distinguish it 

 from that form. It bears a somewhat intimate general likeness to A. 

 ornatus, Ag., and A. semiornatus, McCoy, of the Irish Carboniferous 

 limestone, but may be recognized by its more slender proportions and 

 the downward direction of the denticles in the upper portion of the 

 spine. 



The plainer examples of the present form present also marked resem- 

 blance to the form from the Chester limestone which we have designated 

 under the name AsteroptycMus ? tenuis; the more closely approximated 

 costae and the reversed direction of the posterior denticles are sufficient 

 to enable its recognition. 



It seems not improbable that the reversion in the direction of the two 

 series of denticles iu the postero-lateral angles, as shown in the present 

 species, may constitute a generic feature ; but as the majority of speci- 

 mens only show the upper portion of the spine, it is very rare that this 

 peculiarity is observable. 



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

 Illinois, and St. Louis, Missouri. 



ASTEROPTTCHITJS ? TENUIS, St. J. and W. 

 Tl. XVI, Fig. 5,6. 



Spine of medium size, nearly straight or very slightly curved back- 

 ward, and very gradually tapering from base to apex, transverse sectiou 

 subovate. Anterior angle formed by a simple rounded ridge, postero- 

 lateral angles less acute, with traces of numerous minute denticles sepa- 

 rated by a space above twice their vertical diameter in the middle portion 

 of the spine ; lateral faces slightly arched and beautifully fluted, with 

 seven, more or less, smooth low rounded, parallel, enameled costae, sepa- 

 rated by a shallow depression of somewhat irregular breadth, about 

 twice the diameter of the ridges in the middle, but more erowded 

 towards either margin, the interspaces delicately striated or striate- 

 punctate vertically. Posterior face abruptly truncated, transverse dia- 

 meter about two-thirds that of the lateral faces, median keel somewhat 

 prominent, forming a subacute angle, bordered by wide, gently exca- 

 vated spaces, which are striated in the same manner as the interspaces 

 between the lateral costse. Pulp cavity ovate in transverse section, 



