VERTEBRATES. 93 



Judging from the figures and descriptions before us, it seems to us probable 

 that Cochliodus magnus, Ag., and Helodus planus, Ag., should be separated from 

 their present generic connections and included in Aspidodus. Cochliodus 

 magnus, Ag., corresponds closely in form with some of the teeth now figured, 

 and it certainly wants the characteristic revolving ridges and convoluted form 

 of that genus. It has been suggested by Capt. Jones (Potlock's Geol. Report, 

 p. 642), that Helodus planus is but a form of Cochliodus magnus. 



Aspidodus crenulatus, N. and W. 



PI. VIII, Figs. 3-11. 



Teeth small, flattened thin, showing considerable variety of 

 form and size ; being mostly trapezoidal with five unequal 

 sides, one or more of the sides being sometimes rounded, a por- 

 tion of the series showing a deep notch on one of the sides. 

 Teeth forming several pairs, which occupied corresponding 

 positions on the opposite sides of the mouth. Of these pairs, 

 among the largest and the thickest is one which is obliquely 

 rolled or twisted — as in Cochliodus — is narrow, trapezoidal in 

 outline, with one angle much produced. This was probably 

 the terminal tooth on each side, homologous with the largest 

 tooth in Cochliodus. The upper surface of all the teeth is 

 covered with a thick coating of nearly smooth, uniformly 

 pitted enamel. The articulating edges are strongly crenulated, 

 the inferior surface nearly smooth. 



The convoluted pair of this series o.f teeth bear a marked resemblance to 

 those described by Agassiz, under the name of Cochliodus magnus, and there 

 can be no reasonable doubt that they are generically identical, and yet, being 

 longer, narrower and more revolute, they must be considered specifically distinct. 



It is, perhaps, not quite certain that all the varied forms which we now 

 include in A.< cremdatus should be so united, but the principal pair were evi- 

 dently articulated with others of different form from their own and all the series 

 now figured have as indications of identity the common locality, pentagonal 

 outline peculiarly similar surface and microscopic structure. 



Figures 3-11, represent 12 of these teeth, of natural size; figures 3, 3 a, the 

 most convoluted pair. 



Formation and locality : Chester limestone, Chester, Illinois. 



