456 PALEONTOLOGY OF ILLINOIS. 



truncated posteriorly, lateral faces flattened and nearly parallel iu the 

 posterior half, sharply rounded in front to the anterior margin, and 

 marked in the worn condition by a comparatively deep longitudinal 

 sulcus, slightly anterior to the anterior wall of the internal -cavity. 

 Length along the strongly arched outline .40 inch, lateral breadth near 

 the basal line about .10, greatest thickness iu same region .05 inch. 



The specimen above described may be recognized by its general 

 symmetrical outline and proportions, probably approaching the young 

 spines which we have referred to Ph. gigas, more closely than other 

 determined forms ; but the more strongly arched outline and flattened 

 transverse section offer distinguishing features. It is also less lenticu- 

 lar in transverse section, and probably more gradually tapering than 

 Ph. proclivus of the Kinderhook, with which, in the condition of the sole 

 representation here noticed, it bears intimate resemblance. 



Position and locality : Eare in the Chester formation ; Chester, 111. 



Iu dealing with most of the forms of this genus of ichthyodorulites, 

 we have been compelled, in most instances, to rely upon forms and pro- 

 portions for discriminating features, and these have been given as they 

 appear in the specimens examined, and which it is believed will require 

 little modification, though the discovery of specimens which shall dis- 

 play those other superficial features most essential iu the definition of 

 intimately allied forms will largely add to our knowledge of the affini- 

 ties of the foregoing forms. 



Drepanacanthus reyersus, St. J. and W. 



PI. XIX, Fig. 5,6. 



A fragment of a dorsal spine, discovered by Mr. VanHorne, showing 

 little more than an inch of the inferior portion above the line of inser- 

 tion, appears to be referable to the genus Drepanacanthus. The spine is 

 moderately curved and rather rapidly tapering, in which respect it 

 approaches the Coal Measure form Xystracanthus acinaciformis more 

 closely than D. gemmatus, N. and W., of the Keokuk limestone. Trans- 

 verse section sublenticular, truncated posteriorly, lateral surfaces flat- 

 tened or very slightly convex, gradually converging and rounded along 

 the anterior margin, and occupied by numerous, probably not less than 

 twenty, tuberculose costfe, spaced by less than their own diameter. 

 The tubercles are quite variable iu shape and degree of radiate sculp- 

 turing, varying from vertically to transversly elongated, culminating iu 

 a transverse apical crest over the inferior border, and more or less reg- 

 ularly diminishing in size posteriorly, those of the posterior costa? plain 

 or relatively meagerly sculptured and more widely spaced vertically ; 

 but a marked peculiarity presented by the specimen is the apparent 



