YEETEBEATES. 481 



no trace of spinose processes in the tip, the uniform tuberculation pre- 

 vailing- throughout, though more or less worn near the extremity; but 

 in the concave margin near the extremity occur large tubercles similar 

 to those in the anterior portion of the spine, and which are apparently 

 irregularly disposed. 



Towards the mutilated inferior border, the inner surface of the walls 

 present much the appearance of semi-ossified cartilage, the individual 

 tubercles seeming to have had independent origin in the extreme bor- 

 ders, but later to have become more and more intimately connected by 

 the impingement of their edges, and finally became incorporated in the 

 solid bony basis of the spine, their inner surface no longer showing suture 

 or other signs of isolated origin. Indeed the structure of the base of these 

 spines, as it appears under the ordinary lens, seems to be almost exactly 

 like that of the semi-osseous cartilage of the jaws, remains of which 

 we have discovered in our Lower Carboniferous deposits. Yet, it is not 

 improbable that these bodies present all the phases of developmental 

 origin discernible in ordinary spines, i. e., distinct formative organs by 

 which the basis or inserted portion was formed and subsequently the 

 enameled or external coating was produced, though here we have what 

 at first thought seems to be the anomalous feature of the intimate 

 peripheral association of the organs, but which, in fact, seems to be iden- 

 tical with their occurrence in the development of dormal scutes. 



Position and locality : Upper fish-bed horizon of the Keokuk. lime- 

 stone ; Keokuk, Iowa. 



—62 



