74 FAL7E0NT0L0GY OF ILLINOIS. 



Figure 18, anterior crown face; 18 a, same seen from above; 18 b, from the 

 side. 



Formation and locality: Coal Measures, Posey county, Indiana. 



Genus HELODUS, Ag. 



Gen. Char. — " Transversely elongate, crown convex, eleva- 

 ted along the middle into an obtuse, conical ridge, sometimes 

 divided into a line of several compressed cones diminishing 

 from the centre ; surface porous, as in Psammodus ; margin of 

 the crown raised in the middle on both the inner and outer 

 sides, and it and the root vertically plicated." 



It is evident that among the species at present included in the genus Helodus 

 there is much incongruity, and the whole group requires a thorough revision 

 and more or less sub-division, before anything like a distinct idea can be con- 

 veyed of a generic character represented by the name Helodus. Still the time 

 has, perhaps, not yet arrived, when the required revision can be satisfactorily 

 performed. The number of species which have been described is now about a 

 dozen, and there are in the collection before us at least an equal number, which 

 would be grouped with them in the genus Helodus as at present defined. Of 

 these as we shall be able to show that some are probably nothing more than 

 the median teeth of the jaws which bore on their rami the convoluted plates 

 known as Cochliodus, while others having a general similarity of form, with 

 these apparently constituted portions of a dental series not very unlike through- 

 out. Both these groups have a laterally elongated form, with a conical profile, 

 and a prominent central boss or cone, with or without secondary prominences 

 on either side of this. The median teeth of Cochliodus, a group which must 

 include several described species of Helodus, (e. g. H. Isevissimus, H. didymus, 

 H. turgidus, etc.), have been found in regularly diminishing series like some of 

 those now figured. Another group of Helodi, of which H. planus, Ag., may 

 be taken as a type, in their broad and flattened forms resemble Psammodus, to 

 which they approach more nearly than to the turgid and conical species with 

 which they are now associated. These formed a pavement-like dentition which 

 was, perhaps, similar in character throughout the mouth, or they might have 

 formed a part of the varied dental series of Cochliodus. It is probable, how- 

 ever, that of these a group may be gathered which would possess a fairly 

 defined generic character. It is desirable, however, for the satisfactory classi- 

 fication of this heterogeneous material, that specimens of at least a considera- 



