436 Proceedings of Royal Society of Edinburgh. [sess. 
than in the Ziphioid genera, in the Physeterinoe and Platanista, a 
character which differentiates the Dolphins from the other groups. 
The position and appearance of the ridge on the outer surface 
of the bulla are also of importance. In Physeter it is noteworthy 
from its size, projection, situation near the posterior end of the 
bulla, and extension on to the inferior surface, where it is continued 
into the outer lobe. In Kogia , on the other hand, it is absent. In 
Hyperoodon and Mesoplodon it is not so prominent as in Pliyseter , 
and does not quite reach the inferior surface, and is not so near the 
posterior end of the bulla. In Zipliius , again, it projects slightly, 
and is limited to that part of the outer surface which is in proximity 
to the sharp outer border of the mouth of the tympanic bulla. In 
Plalanista also it has only a feeble projection, is near the mouth 
of the bulla, and about midway between the anterior and posterior 
ends of the surface. In the Delphinidse specified in the preceding 
list it is also placed approximately near the middle of the outer 
surface, is not very projecting, and is limited to the outer surface, 
where it lies close to the sharp outer border of the mouth of the bulla. 
The petrous bone in Kogia, Hyperoodon, Mesoplodon, and 
Zipliius has a smooth plate-like surface of articulation, and is 
adapted to the tympanic by the apposition of two plane surfaces ; 
but in Physeter and the Dolphins as is well seen in Globicephalus, 
the articular surfaces are ridged and furrowed, and are adapted to 
each other somewhat like the denticulated margins of the bones in 
the vault of the cranium. 
( Issued separately June 5, 1903 .) 
