r 
PLATE IX. 
Mosasaurus gigantetjs Sommering, 1816. 
Left quadrate, external side, natural size. Canne (Belgian Limbourg) near 
Maestricht. Upper Cretaceous (Maestrichtian); original in Brussels Museum. 
L. Dollo: Bull. Soc. belg. Geol., 1890, IV, pi. viii, fig. 1. 
Showing the tympanic groove (r. tymp.), indicating a delicate tympanic mem¬ 
brane, and hence a surface-swdmming habitat. 
