INTEODUCTOKY. 



61 



DIAGRAM ILLUSTRATING THE STRATIGRAPHICAL DISTRIBUTION AND THE 

 RELATIONS OF THE GENERA OF COCHLIODONTS. 



Besides the above mentioned Cochliodonts and Psammodonts, a 

 considerable number of heretofore undescribed forms, pertaining 

 chiefly to Ichthyodorulites, or defensive spines, are noticed, being 

 acquisitions acquired since the publication of the preceding volume 

 of the Illinois Geological Survey. As in connection with the latter 

 work the authors owe acknowledgment to the same gentlemen, who 

 have not only contributed material, but have freely shared the 

 results of their familiarity with the objects themselves, and the in- 

 valuable information relating to their stratigraphical association and 

 distribution, so that we feel a double interest with them as con- 

 tributors to a knowledge of these early vertebrates. 



