YEETEBEATES. 435 



diameter of the body of the spine. Spines attain the length of eight 

 to ten inches. Base uuknown. 



Intimately related to A. occidental^, (N. and W. sp.) of the St. Louis 

 limestone, the present form is distinguished from that species by its 

 laterally more compressed figure, the perceptibly narrower and more 

 numerous costse, less erect and more widely spaced denticles along the 

 posterolateral angles. Comparatively few specimens have been iden- 

 tified from collections made by Prof. Worthen, and these are imperfect 

 from accident, though sufficient to show their distinctive features. 

 Much worn examples of Gtenacanthus Keokuk, from the same horizon, 

 bear some resemblance to this form ; but they are readily recognizable 

 by the wider intercostal spaces, and the median keel of the posterior 

 face. 



Position and locality : Not common in the Keokuk limestone ; Warsaw, 

 Illinois. 



Gextts ASTEKOPTYCHIUS, Agassiz. 



ASTEEOPTTCHITJS YETUSTUS, St. J. and W. 

 PI. XVI, Fig. 1. 



A fragment of a small spine, showing less than an inch of the upper 

 extremity, appears to be the earliest representative of the genus Aster- 

 optychius found in our Carboniferous formations. Though imperfect 

 from wear, it differs so unmistakably from its associates in the same 

 horizon, that, taking into consideration its general features, we have 

 little doubt as to its identity with the above genus. 



The spine is long, narrow, or very gradually tapering and moderately 

 curved, thick, and probably subtriangular in transverse section. Late- 

 ral faces gently arched and rapidly converging to the obtusely rounded 

 anterior margin, occupied by at least three narrow, prominent, widely 

 spaced costte, the posterior one abruptly beveled to the posterolateral 

 angles, the costse apparently more crowded posteriorly than in front; 

 intercostal spaces varying from the same to twice the diameter of the 

 ridges, and longitudinally marked by one or two thread like strise, but 

 without any trace of intercostal tubercles. Posterior face faiutly 

 depressed, the channeled area relatively narrower, median keel appa- 

 rently obsolete ; postero-lateral angles moderately prominent, obtusely 

 rounded, armed with relatively large, laterally compressed denticles, 

 spaced by little more than their greatest diameter. Pulp cavity com- 

 paratively large, equaling half the lateral diameter of the spine, poste- 

 rior wall very thin. Base unknown. 



We have thus far examined only the unique example described, so 

 that we have no means of determining the character and extent of indi- 



