272 CHARLES R. STOCKARD AND A. L. JOHNSON 



index and snout index. They again converge to within 50 

 units of one another for the upper facial index and are quite 

 close together for all other features indicated, with the ex- 

 ception of the relation of total size of premolar teeth to the 

 extent of the maxillary and mandibular premolar regions. 

 The reader may follow without difficulty these points along 

 the curves. 



Curves based on the averages for the indices and linear 

 proportions in the F l and F2 skulls lie between the curves 

 for the parent breeds. In the six indices and the relative 

 linear proportions plotted, the hybrid curves almost without 

 exception follow more closely that for the dachshund than 

 for the French bulldog. These results are in close accord 

 with those found for the Boston terrier-dachshund cross, and 

 confirm the conclusion that the various features differentiating 

 these skull types are responses which definitely depend upon 

 a genetic complex of factors having a dominant inclination 

 towards the normal long typed dog skull as opposed to the 

 divergent short and wide bulldog-like skull. The nature of 

 these expressions in relation to the influence of the endocrine 

 systems and other developmental factors must be left for 

 later consideration. 



THE BULLDOG ACHONDROPLASIC SKULL; ITS MODIFIED 

 GROWTH AND DEVELOPMENT 



The discussion of the head and skull features in the two 

 foregoing crosses between animals with bulldog and normal 

 head types makes it necessary, before proceeding further, 

 to inquire into the morphologic nature and development of 

 the aberrant bulldog skull. This is not simply a short, wide 

 skull as compared with the usual type, but one showing con- 

 sistent modifications in many important structural arrange- 

 ments, some of which result in gross disharmonies and evi- 

 dent impairment of function. As mentioned in a previous 

 discussion, the bulldog typed skull is not confined to the dog 

 species alone ; a closely comparable type of skull modification 



