R.-J. ANDERSON — CETACEA ol 
The tympanic is bilobed in Mesoplodon Layardi and not in Zphius. 
(Delphis and Berardius have the same). 
Longest diameter of petrous breadth 
In Ziphius Cavirosta 6,2 D 
Adult Mesoplodon Layardi : Dar A 
Internal meatus of petrous, single in young Wesoplodon, divided into 
two in adult Mesoplodon and Ziphius. 
In Mesoplodon petrous, mastoid and tympanic differentiated but mas- 
toid and tympanic fused, and separated by a constriction. 
