THE GEOGRAPHIC TURTLES. 81 



pace. The. width of the head of an adult M. geographi- 

 cus measured, was contained in the length of the carapace 

 4.68 times. The most surprising difference becomes ap- 

 parent on examining the jaws. The alveolar surfaces are 

 in M. geographicus greatly expanded and when the horny 

 covering is removed the underlying hones are found to be 

 peculiarly modified, the maxillary and palatine of each side 

 with the vomer forming a wide plate with elevated inner 

 margin which almost meets its fellow across the middle line. 



The mandible is correspondingly expanded and flat- 

 tened. Having relation to this enlargement of the jaws is 

 a widening of the skull behind the orbits and a thickening 

 of the supra-occipital process, both having to do with the 

 increased size of muscles necessary to operate the jaws. 



M. lesueuri shows nothing of this expansion of the jaws. 

 The alveolar surfaces are little if any wider than those of 

 Pseudemys elegans, the anterior portion of the vomer is 

 fully exposed, and there is no elevated continuous inner 

 margin formed of palatines, maxillaries and vomer. Com- 

 pared as to the jaws and the bones entering into them, M. 

 lesueuriis more closely allied to Pseudemys or Chrysemys 

 than to M . geograpliicus. I can only account for the fail- 

 ure of the early writers to observe the important differ- 

 ences in these parts by supposing that they did not have 

 adult examples of the two species for examination. 



FOOD. 



The examination of the contents of the digestive tube 

 of M. geographicus throws light at once in the modifica- 

 tion of the jaws and head. 



In all those I examined the food consisted exclusively 

 of mollusks, in the young turtles consisting of Valvata tri- 

 carinata and other thin-shelled species, in the adults of 

 larger and thicker-shelled forms. 



ESSEX INST. BULLETIN, VOL. XXII 6 



