80 



THE DIFFERENCES BETWEEN 



ridge is greater in examples of the latter than in those of 

 M. geographicus, the size of the examples compared be- 

 ing the same. The follo'wing measurements of two ex- 

 amples will illustrate this : 



The median dorsal ridge is a trifle less prominent in 

 adult M. geogropJiicus; in young of the two species not 

 much difference is apparent in this respect. A peculiarity 

 of the ridge of the second and third dorsal plates in M. 

 lesueuri will generally enable one to recognize this species. 

 When viewed from the side the outlines of the ridges of 

 these plates are seen to be concave before the posterior tu- 

 bercles, whereas in M. geographicus the outline is pretty 

 uniformly convex from the base to the posterior extremity 

 of each plate. Most of the bony neural plates in M. ge- 

 ographicus are wider than long ; the reverse is true of M. 

 lesueuri, most of them being in it longer than wide. With 

 these exceptions the carapaces of the two species are much 

 alike. 



In young examples the heads are about equal in size, 

 but an examination of the jaws shows the alveolar sur- 

 faces wider in M. geographicus. The adult M. lesueuri 

 has a rather small head, not larger than that of a Chryse- 

 mys, while the adult M. geographicus has a large head, 

 with jaws quite as massive as those of the snapping tur- 

 tles. The width of the head of an adult of the former 

 was found to be contained 6. 4 times the length of the cara- 



