Ixviii INTRODUCTION. 



size of these teeth distinguish the primary subdivisions of the same 

 Order. A few other groups of Fishes are well defined by dental cha- 

 racters, as the Pycnodonts, Gymnodonts, Goniodonts, and Chsetodonts. 

 But in most of the natural Orders, and in many of the subordinate 

 groups of the Piscine class, the dental system is subject to very great 

 diversity in regard to the form, number, and position of the teeth; 

 and in some natural families there is also a want of constancy in the 

 structure of the teeth. There are extremely few genera of Fishes that 

 can be characterized by a definite numerical dental formula, like 

 most of the Mammalian genera. Indeed, in the first introduction 

 of true teeth into the animal series, regarded in the ascending order, 

 they manifest, like the mouths of the Polypi, the stomachs of the 

 Polygastria, and the generative organs of the Tcenifs, the principle 

 of vegetative or irrelative repetition ; and, in many Fishes, are 

 too numerous to be counted. The limits within which the teeth 

 are applicable as means of classification in Fishes will be readily, 

 and I trust, accurately appreciated, by the descriptions in the first 

 part of this Work. Traced from species to species they are of 

 great importance in the determination of the fossils of this class. 



With regard to microscopic structure, the second and third 

 of the modifications defined in Chapter I, Section 8, are peculiar 

 to and characteristic of the Piscine Class ; the first modification is, 

 with the exception of one Mammalian genus, Orycteropus, peculiar 

 to Fishes ; unvascular or fine-tubed dentine forms the crown of 

 the teeth in a few Fishes, but is more common in those of the 

 higher Classes. 



In the Class Reptilia the teeth serve to characterize smaller and 

 more definite groups than in Pisces, as, for example, the venomous and 

 non-venomous Ophidians ; and the acrodont, pleurodont, and thecodont 

 Saurians. Certain Genera, and even Species may likewise be known 



