80 
THE DIFFERENCES BETWEEN 
ridge is greater in examples of the latter than in those of 
M. rjeograpJticuH, the size of the examples compared be- 
ins: the same. The following measurements of two ex- 
amities will ilkistrate this : 
M, f/cographicus. 
M. lesueuri. 
LENGTH 
OF 
cakapack. 
2.44 iu. 
•2.37 " 
WIDTH OF 
CARAPACE 
OPPOSITE 
THIRD 
DORSAL. 
.25 ill. 
.19 '• 
DEPTH or 
SHELL AT 
MIDDLE OF 
THIRD 
DORSAL. 
1.12 in. 
LOG " 
WIDTH 
OF 
HEAD. 
.50 in. 
.50 " 
AVIDTH OF 
HEAD IN 
ARCH 
ALONG 
RIDGE OF 
CARAPACE. 
4.75 times. 
5.17 " 
The median dorsal ridge is a trifle less prominent in 
adult 31. geograpldcus; in young of the two species not 
much diflereuce 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. ffeorirapliicus 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- 
ogra2)hicus 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. geograplticus. The adult M. lesueuri 
has a rather small head, not larger than that of a Chryse- 
mys, while the adult M. geograpldcus 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 0.4 times the length of the cara- 
