368 paljeo:n t tology of Illinois. 



recognized in the elongated, vertical teeth to which Prof. Agassiz gave 

 the name Chomatodus linearis, (Poiss. Foss. torn. III. Tab. 12, fig. 9j not 

 fig. 5, 7, 12,) the group under consideration, will be found to present a 

 marked contrast in the relative position of the base, which closely 

 adheres to the margin beneath the basal border of the concave crown- 

 face, the basal area beneath the opposite coronal face more nearly cor- 

 responding to that of Antiiodus. In the laterally extended linear out- 

 line of the crown there is strong resemblance to typical Chomatodus; 

 but in the disproportionate depth of the coronal surfaces and the mar- 

 ginal position of the root, it presents affinities with Antiiodus. From 

 the latter, however, it is distinguished by the linear or rectangular out- 

 line of the crown, and perhaps also the distinct definition of the infe- 

 rior surface of the root, which in Antiiodus appears to be rounded as in 

 Petalodus and Ctenopetalus. 



We have, however, observed in certain forms from the St. Louis and 

 Chester formations, an association of individuals the specific affinities 

 of which we strongly suspect, and which exhibit a combination of out- 

 line and contour which might be regarded as evidence positive of their 

 relationship with Antiiodus. But if we restrict Antiiodus to those forms 

 which possess a crown essentially like Petalodus, with a shallow root, it 

 may be admissible to recognize in the above mentioned form a type 

 intermediate between the true Chomatodus and Antiiodus. Tet, if we 

 adhere to the strict interpretation of the facts at present represented, 

 it seems highly probable that the forms referred to under the above 

 head may prove to represent the extremely varied dentition of one and 

 the same genus. 



Of the present type of teeth the following representatives have already 

 been described : Chomatodus gracillimus, N. and W., C. mtdtiplieatus, 

 ~N. and W., of the Upper Burlington ; C. ohscurus, Leidy, of the Keokuk ; 

 C. angulatus, jST. and W., of the Coal Measures. In the study of the 

 collection of Mr. Speingee, we have been led to suspect the possible 

 specific identity of such forms as represented by C. gracillimus and a 

 form closely resembling Antiiodus politus, 2ST. and W., which it repre- 

 sents in the Upper Burlington deposits. Should these inferences prove 

 correct, then all the forms hereinafter noticed will probably fall under 

 the genus Antiiodus, as amended and extended. 



Tanaodus sublunatus, St. J. and W. 



PI. XI, Kg. 27. 



We have provisionally recognized the specific distinctness of a form, 

 represented by a single specimen in the collection of Mr. VanHoene 

 from the St. Louis, which bears certain resemblances to the teeth 



