THE GEOGRAPHIC TURTLES. 8l 
pace. The width of the head of tin adult M. (jeorjrai^ld- 
ais 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 31. geograpJiicus greatly expanded and when the horny 
covering is removed the underlying bones 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 thickenin"; 
of the snpra-occipital process, both having to do with the 
increased size of muscles necessary to operate the jaws. 
M. lesueuri shows nothing df 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, 31. 
lesueuri is, more closely allied to Pseudemys or Chrysemys 
than to 31. geograpJiicus. 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 disfestive tube 
of 31. geograpJiicus throws light at once in the modifica- 
tion of the jaws and head. 
In all those I examined the food consisted exclusively 
of mollusks, inthej^oung turtles consisting of Valvala tri- 
carinata and other thin-shelled species, in the adults of 
larger and thicker-shelled forms. 
ESSEX INST. BULLETIN, VOL. XXII 6 
