74 THE MAMMALIA OF THE DEEP EIVEB BEDS. 



generic or even ordinal reference, as it is uncertain whether they are carnivores or 

 creodonts, but it is altogether likely that they will prove to be intermediate between 

 Daphcenus and the Mtacidce of the Bridger. 



The great difficulty in the way of making out a satisfactory phylogeny of the 

 Canidce is the position to be assigned to the problematical genus Otocyon. If, as so 

 many authorities maintain, it he inadmissible to assume that in this form the number 

 of teeth has been increased at a comparatively recent period and within the limits of 

 the family, then we shall be driven to admit a very remarkable degree of parallelism, 

 or rather of convergence. Either the series of fossil forms which lead by slight and 

 natural gradations from the Miaciche to Cams have nothing to do with existing spe- 

 cies, but merely form a parallel series, leading to no permanent result, while the real 

 ancestors of the family are entirely unknown; or, on the other hand, Otocyon must 

 represent the teimination of a line leading upwards from some creodont family, as 

 yet undiscovered, which line has paralleled the dogs in every detail of structure 

 except the dentition. For my own part, I am by no means convinced of the impos- 

 sibility of the addition of new teeth to the molar series. That modification in the 

 mammalian lines is very generally by way of reduction in the number of teeth, is 

 true, but does not prove that the reverse process may not exceptionally take place, 

 whether by reversion or otherwise. The great simplicity of the teeth in Otocyon can 

 hardly be reconciled with its advance in all other respects, except on the hypothesis 

 of a retrogression or reversion in dental structure. At all events, such an assump- 

 tion would seem to involve less of improbability than either born of the dilemma to 

 which its rejection confines us. 



Stress has been laid upon the lyrate sagittal area of Otocyon and its occurrence 

 in the young of other species of the family as showing that it is a primitive charac- 

 ter. But an examination of a series of fossils in almost any mammalian phylum 

 shows that the high and thin sagittal crest is the primitive character, and its replace- 

 ment by a flattened area the secondary modification. The reason for this is plain ; in 

 the ancient forms, the jaws and canine teeth are powerful and the brain is small, 

 hence the cranium does not offer sufficient surface for the attachment of the tem- 

 poral muscles, and the sagittal crest must be developed, just as in the analogous case 

 of the sternal keel in birds. Now, the disproportionately large size of the brain in 

 the young animal gives a large surface for muscular attachment at a period when the 

 weak jaws and small milk teeth require little muscular power, and hence the devel- 

 opment of the crest is retarded. In no embryonic structure are there so many 

 " cenogenetic " features as the skull, just on account of the great and premature 

 enlargement of the nervous axis and the higher sense organs, and hence embryologi- 



