YEETEBEATES. 433 



fossils, and of the same kind ; it is apparent, therefore, in the present 

 state of our knowledge of the facts, that it is matter of individual 

 opinion merely, and that we must seek a fuller acquaintance with facts 

 to be derived only in the field where we may carefully note the occur- 

 rence and association of these fragmentary remains, before we can hope 

 to satisfy ourselves in regard to their generic and specific identity. 

 Although in several instances our American collections have been made 

 with the utmost care, in the hope of securing every fact that might 

 throw the least light on the affinities of the remains associated in the 

 same beds, our data do not furnish unequivocal evidence by which the 

 generic identity of fin-spines and teeth may be determined. Many sug- 

 gestions may be derived from the study of this splendid material ; but 

 ■where suggestions are so conflicting their reliability is questionable. 



In relation to the identity of the Carboniferous spines under consid- 

 eration with the Mesozoic Leptacawthi, judging from the descriptions of 

 Professor Agassiz and the illustrations accompanying the Poissons 

 Eossiles, there is apparent in the distinctive features of the more ancient 

 spines the uniform deep groove of the posterior face, the Liassic Lepta- 

 canthus being nearly plane across the posterior face; the costation is 

 apparently very similar in the spines from the widely removed geologi- 

 cal positions, and the denticulation of the posterior borders is equally 

 similar. 



The genus embraces the following described species, all of which per- 

 tain to the Carboniferous period : Leptacanthus junceus, McCoy, L. Jen- 

 Mnsoni, McCoy, Cladodus tenuistriatus, Eomanowsky, Leptacanthus 

 occidentalis, ~N. and W. 



ACONDYLACANTHTJS GRACILIS, St. J. and W. 

 PL XVI, Fig. 8-11. 



Fin-spines of small or medium size, probably attaining the length of 

 four or five inches, laterally compressed or sublenticular in transverse 

 section, gradually tapering in the lower and middle portions, but rap- 

 idly converging near the apex, which forms an acute point. Lateral 

 faces gently arched, flattened posterior to the middle, and regularly 

 converging in front to the more or less sharply rounded anterior mar- 

 gin, and occupied by a few, about ten, relatively strong, irregular costaj, 

 which increase by infrequent bifurcation descending, the anterior ridge 

 sending off more frequent branches near the apical extremity, and quite 

 regularly spaced by narrow, shallow intercostal furrows ; the costse are 

 low and flattened or gently rounded along the crest, and generally more 

 delicate along the posterior border, though their development varies 



—56 



