— 327 — 
brasiliensis the tibia, relatively to the femur, is a little longer. 
The form of the leg bones is well shown in Plate IIL, fig. 5 — 
the tibia heavy proximally, small distally; the fibula flattened, 
broad, and curved away from the tibia. Distally these bones are 
wedged wide apart by the large intermedium, which forms a complex 
with the tibiale. The tibia articulates with the tibiale portion of 
this complex, the fibula with the intermedium portion and also 
with fibulare. The blood-vessel foramen between intermedium and 
fibulare, shows clearly as in Stereosternum, and the five distal 
tarsals (the diagnostic character of Baur’s order Proganosauria, 1887, 
p. 9) are also similar to those of that genus. The first and fourth 
are slightly larger than the rest. From the material at hand I have 
not been able definitely to distinguish any difference in the hind 
feet in the two genera. The first metatarsal is the heaviest, and 
there is a regular increase in length to the fifth, or post-axial digit, 
which is nearly twice the length ofthe first. In Stereosternum the 
fifth is almost exactly twice the length of the first, and in Broom’s. 
figure (1904, Pl. IX., fig. 5) of the South African Mesosaurus it is 
more than double. In all the figures of the Mesosaurian pes given 
by Cope, Seeley, Osborn and Broom, the fifth digit is represented as 
curved slightly outward, and separated from the fourth by a greater 
interval than that between the other digits. The similar position 
and regular spacing of the digits in so many specimens almost 
certainly indicates a swimming web, as Osborn suggested (1903, 
p. 489). The phalangeal formula of the pes is 2, 3, 4,5, 3, and the 
terminal elements, as in the manus, apparently bore only vestigial 
claws. The great size of the pes in comparison to the manus is very 
striking. Undoubtedly the former served as a highly efficient propel- 
ling organ in swimming, while the forelimb probably functioned 
chiefly as a balancer. It is my opinion that these animals were almost 
wholly aquatic in habit, and ill-adapted for motion on land, though 
undoubtedly descended from land-living ancestors. 
Abdominal ribs, etc. 
The plastrom of dermal ribs extends from near the shoulder 
girdle to the pelvis. Its component elements (for the most part 8 to 
143 ™™, Jong) are placed closely together and overlapping, and are 
less regularly arranged in series — than in some reptiles. Longitudinal 
lateral series are readily identified, but whether there is a single 
