238 PALEONTOLOGY OF ILLINOIS. 



The spine under consideration is, perhaps, less intimately allied 

 to Ctenacanthus gemmatus, St. J. and W., than with C. deflexus 

 herein described, both from the same formation. In certain features 

 of surface ornamentation it resembles the former, but in general 

 form it is apparently allied to the latter; while in the details of 

 ornamentation it possesses features readily distinguishing it from 

 the described species of -the genus. The fragment here alluded to 

 is a rare acquisition at the locality where it was found, which has 

 thus far afforded very few specimens of ichthyodorulites. 



Geological position and locality: St. Louis formation, calcareous 

 shales ; Pella, Iowa. 



CTENACANTHUS GKACTLLIMUS, N. and W. 



PI. XXIV, Fig. 1. 



Ctenacanthus gracillimus, NEWBEEET and WOETHEN, 1866, 111. Geol. Survey, II, p. 126, 

 PI. XIII. f . 3. 



Leptacanthus? occidentalis, NEWBEBBY and WOETHEN, 1866, ib., II, p. 116, PI. XII. f. 2. 

 Acondylacantkus occidentalis, ST. JOHN and WOETHEN, 1875, ib., VI, p. 433. 



The above species is, perhaps, the most frequently met with of 

 all the ichthyodorulites occurring in the St. Louis formation. The 

 fine example now illustrated, and which w r as kindly loaned us for 

 examination by Dr. George Hambach, belongs to the Shumard col- 

 lection in Washington University at St. Louis, and is unquestionably 

 the finest specimen of the species as yet discovered. 



It is interesting to note the wide distribution of this ichthyodorulite. 

 We have been favored with the loan of a specimen from equivalent 

 deposits at Grand Eapids, Michigan, by Prof. E. A. Strong, which 

 undoubtedly is identical with the present species. 



The fin-spine originally described under the name Leptacanthus? 

 occidentalis, N. and W., and subsequently referred by the authors 

 to the genus Acondylacanthus, we are all but convinced is not spe- 

 cifically distinct from the above species. The latter spines are 

 doubtless the worn upper portion of that species, in which the 

 tuberculation of the anterior or dorsal keel and the first few lateral 

 costiB are obsolete; the costae themselves and the character of the 

 posterior denticles are undistinguishable from that obtained in the 

 two fine examples of Ct. gracillimus, above particularly referred to. 



Geological position and localities: St. Louis formation ; St. Louis, 

 Missouri, Alton, Illinois, Grand Kapids, Michigan. 



