106 
ANNALS NEW YORK ACADEMY OF SCIENCES 
Mosasaurus Conybeare, 1822. 
Plioplatecarpus Dollo, 1882. 
Swimming Mosasaur. 
Diving Mosasaur. 
Surface. 
Depths. 
Size up to 15 meters. 
Size up to 5 meters. 
1 . 
Teeth strong. 
1 . 
Teeth weak. 
2. 
Orbits lateral. 
2. 
Orbits facing upwards. 
3. 
Parietal foramen reduced. 
3. 
Parietal foramen well developed. 
4. 
Tympanic membrane delicate. 
4. 
Tympanic membrane calcified. 
5. 
No median basioccipital canal. 
5. 
A median basioccipital canal. 
6. 
Thorax long. 
6. 
Thorax short. 
7. 
Caudal fin powerful. 
7. 
Caudal fin w T eak. 
8. 
Anterior paddles small. 
8. 
Anterior paddles strong. 
L. Dollo, Bull. Soc. belg. Geol., 1904, xviii, 207; ibid. 1905, xix, 125. 
The calcified tympanic membrane of Plioplatecarpus is an adaptation 
for resisting the temporary heavy pressure of the water at great depths. 
A different arrangement, but serving the same purpose, is seen among the 
Cetacea, which are able to dive to 1000 meters below the surface. 
The median basioccipital canal of Plioplatecarpus is an adaptation to 
protect the large arterial trunks supplying the cerebral circulation from the 
temporary heavy pressure at great depths. A different arrangement, serving 
the same purpose, is found among the Cetacea. 
In Mosasaurus the caudal neurapophyses (spines) and htemapophyses 
(chevrons) are long, the latter being coossified with the vertebrae (centra). 
In Plioplatecarpus the caudal neurapophyses and hsemapophyses are short 
and the latter are separate from the vertebree (centra). 
Provisional Correlations. 
1. Iguanodon (Lower Cretaceous) corresponds best with Claosaurus 
(Upper Cretaceous). 
2. Mosasaurus corresponds with Clidastes. 
Hdinosaur us “ “ Tylosaurus. 
Prognathosaurus “ “ Platecarpus. 
Plioplatecarpus “ “ ? 
3. Orthomerus (Maestrichtian) corresponds best with Trachodon (Laramie), 
but Champsosaurus has already appeared in the Laramie in North 
America, while in Belgium it does not appear until the Lower Eocene. 
4. The upper Senonian witnessed the culmination of the Mosasaurs in 
Belgium, since there were at that time four contemporaneous genera 
while in the Lower Senonian there were but two, and the same number 
in the Maestrichtian. 
