20 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



representing as they do merely the specialization peculiar to Arthrodires. 

 Their aggregate is of no greater taxonornic importance than the sum- 

 total of differences between Ctenodipterines and Sirenoids. Quite the 

 contrary, for the distinctions between Arthrodires and Ceratodonts 

 are on the whole less trenchant than between the latter and Cteno- 

 dipterines. The one constant character in which all Ctenodipterines 

 differ from existing Dipnoans is, as pointed out by Bridge, the multi- 

 plicity and almost Acipenseroid arrangement of their cranial roofing 

 bones. Oddly enough, it is precisely this feature wherein a constant 

 difference exists between the groups named, that a constant resemblance 

 is to be noted between modern lung-fishes and Arthrodires. The 

 cranial pattern of the two latter types is essentially identical, but anom- 

 alous as compared with all other vertebrates. 



The differences between the Ctenodipterine and Sirenoid orders of 

 Dipneusti have been tabulated by several writers, among whom it will 

 be sufficient to mention Bridge and Ftirbringer, in their monographs 

 already several times quoted. By extending the range of comparison 

 far enough to include Arthrodires as well, it will be observed that the 

 two extinct orders (i. e., Ctenodipterines and Arthrodires) agree with 

 modern Dipnoans and differ from all other fishes in possessing the fol- 

 lowing combination of cranial characters * : — 



1. The presence of characteristic tritoral or trenchant dental plates in 

 upper and lower jaws, the former supported by palato-pterygoid ele- 

 ments, usually ossified, and the mandibular by the greatly developed 

 splenial bones of the lower jaw. 



2. The absence of maxillae and premaxillae in the upper, and of a 

 true dentary bone in the lower jaw. 



3. The presence of only two opercular bones, an operculum and an 

 interoperculum, and the absence of a distinct preopercular element. 



4. Complete and typical autostyly in at least the two more com- 

 monly recognized orders, and presumably in the Arthrodiran as well. 



Structural Characters of Mylostomids. 



Mylostoma was established by J. S. Newberry in 1883 upon the 

 evidence of dissociated parts of the dentition, no specimen being known 

 to him in which the dental elements were preserved in natural position 

 or accompanied by other portions of the skeleton. Under these cir- 



1 See the remarks on this subject by Professor Bridge, in his Monograph on 

 Lepidosiren, page 365. 



