O PALAEONTOLOGY OP OHIO. 



plate on either side. This seems to me, however, a less important feature 

 than he regards it, and the argmnent used to sustain the view cited above, 

 viz. : that " teeth never coalesce," is hardly supported by facts. Even if 

 true, it could have no bearing on this question, as the organs under discus- 

 sion cannot be accurately called teeth. 



Among the bones of the head in fishes, we find a very wide range of 

 variation, in number, size, position, and mode of union, ai.d this where the 

 homologies can be clearly made out. N^ow we must suppose each phase 

 of variation to be the result of a special creation, or that the various modi- 

 fications are derived one from the other. The teeth of fishes are far more 

 variable than the bones proper, and many instances might be cited in 

 which the diversity of size, number, and form of the teeth has apparently 

 resulted from fission or union. A good illustration of this is seen in the 

 genus CoohUodus, where some of the species differ mainly in this, that 

 in one (e. g., C. contortus) the scroll-like teeth are composed of sev- 

 eral rings, set side by side, while in others (as G. nobilis), they are 

 united in a solid cylinder. Perhaps a still better example is afforded by 

 the genera Dipterus and Ueliodus. These are closely allied, and they 

 are of special interest in this connection, as they are the ancient represent- 

 atives of the group of Dipnoans to which the Australian Barramunda {Cerio- 

 todus Forsteri) belongs. In Dipterus, there are two triangular fanlike 

 teeth set on the palato-pterygoid bones. These are in contact by the longer 

 of the sides which inclose the right angle, but are not united. In Helio- 

 dus, these two teeth are completely fused into one (see Plate LVIII., 

 Figs. 15-18). 



Where the general plan of dentition is so distinctly preserved, as it is 

 through phases of variations similar to those mentioned above, it is im- 

 possible to resist the conclusion that these phases have had a common 



Dentition of Diniohthzs Hebizebi. 



Front view (diagram) one-teatb natural size, linear. 



The question of the homologies of the " premaxillaries " oi Diniohihys— 



