VERTEBKATES. 187 



Only three examples of the aboved escribed form occur in the collec- 

 tions, and of these only one displays the entire outline and general 

 coronal contour. The latter example was discovered by Mr. L. A. 

 Cox, in the rock excavations for the government canal around the 

 Keokuk rapids on the Mississippi, and to whom we also owe the 

 opportunity to examine additional material illustrating the dentition 

 of this extraordinary species. We have little doubt as to the 

 specific relations of the above described form, and from its peculiar 

 outline and coronal contour, and the strong spirally inrolled ex- 

 tremity (which makes a complete revolution, then becomes involved 

 in the vaso-dentine, forming a solid mass except for the minutest 

 perforation intercommunicating with either border through its 

 center), we are led to identify it with the mandibular median form 

 of this genus. The specimen affording the above measurements 

 doubtless belonged to a mature fish ; the middle portion of the 

 crown, indeed, is excessively worn, affording a striking illustration 

 of the severe usage to which the teeth were subjected during life. 



It is our belief that the form originally described under the name 

 Deltodus grandls, N. and. W., is specifically identical with Sandalo- 

 dns lavissimus, N. and W., which latter name was applied to the 

 long teeth .which we presume to have held the posterior place 

 upon the upper jaw, the former form occupying the opposed posi- 

 tion upon the mandible. The meagre data might not be deemed 

 as affording sufficient grounds for the limitation of the three above- 

 mentioned forms ; but the facts themselves, though few, seem to us 

 to offer something more than a mere suggestion of such a relation- 

 ship. The names 'Sandalodus grandls, N. and W., Cochliodus ? cras- 

 sus, N. and W., Psammodus semi-cylindricus, N. and W., and Psam- 

 modus '} rhomboideus, N. and W., were applied to abnormally worn 

 and otherwise imperfect examples of the maxillary posterior form 

 of this species. 



Geological position and localities: Keokuk limestone ; Keokuk and 

 Bentonsport, (Iowa,) Hamilton, and Warsaw, (111.,) Boonville, (Mo.). 



SANDALODUS, sp.? 



PI. XII. Fig. 5,6. 



The Warsaw limestone has thus far afforded very meagre and 

 imperfectly preserved material referable to the genus Sandalodus. 

 The Warsaw horizon at the mouth of the Piasa, above Alton, has 



