182 IOWA GEOLOGICAL SURVEY 



pterygoid cartilage was of the usual pattern found in all Dip- 

 noans. This cartilage, when ossified, is commonly known as 

 the "upper dentigerous bone"; the fact that it is unossified in 

 Arthrodires agrees with other evidence pointing to their less 

 highly specialized condition as compared with Ctenodipterines. 



The restoration of the upper dentition of M. variabile, shown 

 in the above figure, is based upon the naturally associated parts 

 of a single individual — the same which has already been care- 

 fully described in Dean 's memoir of 1901 — and is consistent with 

 the evidence obtained by fitting together of detached specimens 

 belonging to the same species. It may be instructively com- 

 pared with text-figure 23 (see page 162), which is reproduced 

 from a photograph of the actual dental plates. The retention 

 throughout life of two pairs of palato-pterygoid dental plates 

 in this family, corresponding to an evanescent stage in Neocer- 

 atodus, is regarded as a primitive characteristic. Hence, in so 

 far as the dentition is concerned, members of this family recall 

 ancestral conditions more distinctly than either Dinichthys or 

 Coccosteus. 



In working out the above arrangement of Mylostomid denti- 

 tion, upper and lower, the writer has used all available material 

 illustrating the palatal dental plates of the type species ; and in 

 the whole number of specimens examined, absolutely no char- 

 acters can be detected which point to more than individual 

 differences between them. So close is their agreement with one 

 another in form and relative proportions that is it quite im- 

 possible to suppose, or at least to prove, that more than a single 

 species is represented. Their impact against the lower dentition 

 has given rise to facets and worn surfaces which are seen to 

 occupy a constant position in all the plates, and furthermore 

 to coincide perfectly with similar indications of wear in the 

 lower dental plates when the latter are applied against the pave- 

 ment teeth in their inferred natural position. In this position 

 alone is there harmonious adjustment between all mutually op- 

 posed parts, and in no other position can all of the salient 

 points of contact and worn areas be brought together when 

 the jaws are closed. An arrangement in which all of the parts 

 fit thus perfectly together, and which is capable of explaining a 



