10 bulletin: museum of compakative zoology. 



they had always been found in the detached condition, without being 

 associated with a Dinichthys-like mandible or other parts suggestive of 

 Arthrodires, no one would question the propriety of referring them to 

 Dipnoans. In general form, mode of attachment, and microscopic struc- 

 ture, they differ in nowise from the characteristic palato-pterygoid denti- 

 tion of lung-fishes. If it be objected that they have neither tuberculated 

 surfaces nor crenulated outer margins, it must be remembered that 

 various Dipnoan genera are known which have perfectly smooth dental 

 plates. The plates themselves, therefore, reveal no characters which 

 enable us to distinguish them from the Dipnoan type of upper 

 dentition. 



It has been demonstrated, however, in the most convincing manner, 

 that these upper tritoral plates of Mylostoma belong to Arthrodiran 

 fishes essentially like Dinichthys, except that the dentition is adapted for 

 crushing instead of cutting ; in other words, the two genera mentioned 

 present an interesting parallel, as regards their dentition, to the modern 

 Neoceratodus and Protopterus. It follows as a matter of course that the 

 jaw-parts of the two recent and the two fossil genera must be respec- 

 tively homologous, inasmuch as common ancestry is implied for the 

 members of eitlier pair. Great as may appear at first sight the differ- 

 ences between the Mylostoma and Dinichthys form of dentition, they 

 are nevertheless reducible to a common plan, and this common plan is 

 identical with that typified by Neoceratodus. In the first place it is to 

 be noted that the two Arthrodiran genera under comparison have a 

 single, and somewhat similar, pair of vomerine teeth, as do also modern 

 Dipnoans. Next it will be observed that a like form of mandible is 

 present in both genera, bating only that in Mylostoma the dental plate 

 lies horizontally expanded, and in Dinichtliys it is turned vertically, so 

 that what was formerly the denticulated outer margin now becomes the 

 functional cutting edge. There remains finally the palatal dentition to 

 be considered, and here the fact requires explanation that two tritoral 

 pairs of dental plates occur behind the vomerines in Mylostoma, as 

 opposed to the single pair of "shear-teeth" in Dinichthys, which have 

 already been interpreted as the morphological equivalent of the palato- 

 pterygoid dental plates of Ceratodonts. Enlightenment on this point is 

 furnished by the ontogeny of Neoceratodus, which teaches that the dis- 

 crepancy is more apparent than real ; for, as shown by Semon,^ the dental 

 plates of the modern genus arise through concrescence of conical denticles, 

 which are at first disposed so as to form two pairs of palato-pterygoid 



1 Loc. cit., 1899, and 1901. 



