No. 7. — Mylostomid Dentition. By C. R. Eastman. 



The reconstruction of the Mylostomid type of dentition acquires sig- 

 nificance through its relevancy to the larger question of the affinities of 

 Arthrodires. Nature has not disclosed to us by direct evidence the 

 manner in which upper and lower dental plates of Mylostomids 

 functioned against one another during life. The disposition of the 

 various parts must therefore be determined by indirect means, such as 

 by observing evidence of co-adaptation, mutual contact and wear, and, so 

 far as may be, through analogy with related forms. In reality the 

 problem is a simple one, devoid of mystery and intricacy, and requiring 

 little mechanical ingenuity for its solution and complete verification. 

 Of trivial intrinsic importance, its solution promises enlightenment as to 

 the relations of the perplexing group of Arthrodires. A matter of minor 

 interest in itself, it determines consequences of real magnitude, and 

 hence is worthy of thoughtful consideration. It is proposed in the 

 following pages to examine into the general nature of the problem, 

 the different solutions that have been proposed for it, and some of the 

 consequences depending thereon. 



The limiting conditions of the problem may be stated first. Mylosto- 

 mids are known upon the evidence of two fairly well-preserved skeletons 

 to be Arthrodiran fishes essentially like Dinichthys, except that their den- 

 tition is adapted for crushing instead of cutting. The two specimens re- 

 ferred to are the only ones thus far discovered which present us with the 

 disarranged but nearly complete dentition of single individuals. The 

 fact that in each case the dental elements are known positively to have 

 belonged to a single individual not only facilitates their reconstruction, 

 but furnishes a scale of relative proportions which may be presumed to 

 hold constant throughout the species. Thus provided with a standard 

 of comparison, we may select from a sufficiently large assortment of 

 detached plates the necessary components of a complete dentition, all 

 of whose parts shall be proportionate with respect to one another, and 

 shall have precisely the same conformation as those known to have been 

 associated in a single mouth. Or, given a detached mandible of the 

 same configuration as those found in natural assemblage with other parts, 



