VEETEBEATES. 171 



therefore, it is evident the form held a median, possibly an anterior, 

 position probably on the mandible. It remains to ascertain its 

 probable associate forms, and here again we are met by the insuf- 

 ficiency of .our understanding, which hardly more than suffices 

 vague suggestion. All the individuals alike of all the varieties ex- 

 hibit a peculiar broad undulate uneven course in the borders of the 

 teeth, which toward the outer inrolled margin usually takes the form 

 of a more or less distinct sudden constriction or narrowing of the 

 coronal region. A very similar character is noticeable in the form 

 previously described by Messrs. Newberry and Worthen under the 

 name Cochliodus costatus, "second tooth," in which the lateral bor- 

 ders are similarly widely undulate and narrowed towards the outer 

 margin, where the strongly inrolled extremity has lost its coronal 

 enamel presenting the appearance of having been embedded in* the 

 integument of the jaw, precisely as observed in the form here par- 

 ticularly noticed. Further, there is a somewhat striking resemblance 

 between the latter forms in respect to the peculiar character of the 

 punctation and the rugged surface of the crown, and the relative 

 great depth of the lateral border, although in the present form, 

 apparently, the depth of the borders is even proportionately greater 

 than is the case in "C. costatus" ("second tooth). Beyond this we 

 have not the materials for a more satisfactory comparison, but it 

 seems to us not improbable the latter forms originally formed parts 

 of the dentition of one and the same species. 



Geological position and localities : Upper Burlington limestone ; 

 Burlington, Augusta, Buffington creek, Louisa Co., Iowa; Keokuk 

 limestone; Warsaw, 111. 



GENUS TOMODUS, Agassiz. 



PSAMMODUS, Agassiz, 1838, Poissons Fossils III. ; TOMODUS, Agassiz, 



Mss. ; 1859, etc. 



The genus Tomodus, Agassiz, founded upon the species T. con- 

 vexus of the British Mountain limestone, is known to us by one or 

 two imperfect examples of the large strongly arched form contained 

 in the collections of the Museum of Comparative Zoology, Cam- 

 bridge, which, however, do not afford so satisfactory means of com- 

 parison as might be desired in the definition of their generic char- 

 acteristics. We are, however, indebted to the courtesy of Lord 



