104 



GIDLEY 



lambda Osborn, seems to indicate a third source from which the 

 hypocone may have developed. In Protolambda the internal 

 heel (protocone) is broadly expanded and flattened posteriori}'' 

 without a cingulum, yet the peculiar shelf-like form of this por- 

 tion of the tooth suggests the origin of a hypocone budding off 

 from the protocone independently of either the cingulum or pos- 

 terior intermediate cusp. 



From such a form as that presented in PL V, fig. 7, it is but 

 a short step to the typical selenodont artiodactyl type of molar 

 through the progressive development of the V-shaped posterior 

 intermediate cusp. The addition of a second posterior cusp 

 budding off from the enlarged postero-intermediate cusp would 

 readily convert the tooth into a perissodactyl type of molar. 

 Thus is suggested a fourth possible source of origin for the 

 hypocone. This does not necessarily imply an actual relation- 

 ship of this particular form to the ungulates, but indicates a 

 type closely resembling them which differs widely from the 

 primitive carnivores and insectivores, in which the hypocone, 

 when present, was undoubtedly derived from the cingulum. 

 These observations suggest especially that apparently homol- 

 ogous elements in the teeth of the more highly complex forms 

 may often arise from different sources. 



The correlation and homologies of the cusps of the lower 

 molars in comparison with those of the upper series have, for 

 the most part, been left out of this discussion. One observa- 

 tion, in this connection, however, of seeming great importance 

 and significance should be noted here. 



In examining a large number of examples of both living and 

 extinct forms, I have found the following associations between 

 the heel of the lower molars and the protocone of the upper 

 teeth to hold constantly true, viz : A functional, broad, crush- 

 ing protocone is invariably associated with a well-developed 

 crushing heel in the opposing lower molar. A reduced or vesti- 

 gial protocone is invariably associated with a correspondingly 

 reduced or vestigial heel in the opposing lower molar. Since 

 the heel of the lower molars is admittedly of secondary origin, 

 this feature alone would seem to argue stroncrlv for a like sec- 

 ondary origin for the protocone in the upper molars. 



