268 bulletin: museum of comparative zoology. 



the efficiency of the critical test here employed, we have merely to satisfy 

 ourselves that the new form of compound plate made known by Dr. 

 Hussakof is the exact morphological and also functional equivalent of the 

 anterior pair of palatal plates in Mylostoma. 



We will not now undertake to prove this equivalency of parts; but let 

 it be granted, for sake of argument, that it exists. The immediate con- 

 sequence is that one of the three pairs of upper dental plates in Mylosto- 

 mids becomes definitely fixed in position ; and its allocation necessarily 

 serves at the same time to fix the position of the other two. We are 

 thus pi-ovided with means for distinguishing absolutely between vomerine 

 and either of the two succeeding pairs of palatal teeth, besides being able 

 to assign each pair to its normal position with respect to the others. 

 In addition, we are enabled to establish definite homologies between the 

 several components of Mylostomid and, let us say, Dinichthyid dentition 

 — or even the dental apparatus of other groups of Lung fishes. 



Again, in the light of the foregoing discussion, it must be acknowl- 

 edged that among different genera of Mylostomids the full complement 

 of upper pavement plates may exist as discrete entities (e. g., Dinomylos- 

 toma), or in certain forms the total number may be more or less reduced 

 by fusion. Dinognathus illustrates fusion between the members of the 

 anterior pair of palato-pterygoid dental plates. Up to the present time 

 no Mylostomid genus is known which displays the two pavement plates 

 on either side of the mouth fused into a single piece. Yet there is no 

 iuherent improbability that such a condition of affairs might exist, and 

 as a matter of fact this is the identical condition which does exist — 

 among the Dinichthyid group of Arthrodiran fishes. The evidence of 

 Dinognathus regarding the extent to which fusion is capable of taking 

 place among the upper dental elements of Arthrodires is confirmatory 

 of the view of an exact homology between the so-called " shear-tooth " 

 of Dinichthys and the two pairs of tritoral pavement plates in Mylosto- 

 mids. Unquestionably the similarly formed and similarly disposed 

 vomerine teeth of Dinichthys and Dinomylostoma stand in perfect 

 mutual correspondence ; and furthermore the Dinichthyid " shear- 

 tooth " corresponds to the two pairs of Mylostomid pavement teeth. 



The objection has been urged, however (Science, July 12, 1907), that 

 the Dinichthyid " shear-tooth " is a single large plate, and proof is want- 

 ing that it is of compound nature, or arose from fusion of two 

 distinct elements. But this argument does not hold in the light of 

 new evidence that has fortunately come to hand in Dinognathus. For 

 if two primarily distinct elements can unite in the manner revealed by 



