1902-3.] Sir William Turner on the Sperm Whale. 
435 
It will be observed that it is the rule for the tympanic bulla 
to be both longer and broader than the petrosal bone, but in 
Hyperoodon the contrary is the case in a marked degree. The 
bones differ in size in the several species clanculus, Phocoena , D. 
delphis , and Kogia ; the smallest species have the smallest bones, 
and they attain their greatest magnitude in Physeter, Ziphius , and 
Hyperoodon. But notwithstanding the huge bulk of the sperm 
whale, its tympano-petrous bones are only slightly larger than 
those of the much smaller Hyperoodon , and are much less than 
those of the whalebone whales ; even Balcenoptera rostrata , the 
smallest of our northern species of Mystacoceti, though scarcely 
half the bulk of the sperm whale, exceeds it in the size of its 
tympanic bullse. 
In the Ziphioid genera Hyperoodon and Mesoplodon , which are 
the nearest allies of the Physeterinse, the tympanic bulla is bilobed 
posteriorly, the outer lobe is larger than the inner, and the inter- 
mediate groove is relatively shallow, as in Kogia and Physeter. In 
Ziphius , on the other hand, which gives its name to the group, 
the posterior end of the bulla is not bilobed, but an imperfect 
groove is present at the posterior part of the inferior surface of the 
bulla ; and although it marks off a definite lobe on the outer side, 
the bone immediately internal to the groove is not prolonged into 
a lobe, and the bulla differs therefore materially in its character 
from that of Mesoplodon , l 
In Platanista the bulla is bilobed, and as usual the outer lobe is 
larger than the inner. A broad groove separates the lobes and 
extends to near the anterior end of the inferior surface, which is 
attenuated to a fine point. About the middle of this groove a 
roughened ridge, 18 mm. long, divides it into two parts. In the 
Delphinidse the bilobed character of the bulla is strongly marked 
in Monodon , Phocoena , Globicephalus melas and macrorhynchus y 
Grampus griseus , Lagenorliynchus albirostris and clanculus , 
Delphinus delphis and tursio : and in these specimens the groove 
between the lobes, more especially in Monodon , is much deeper 
1 I have given figures of the tympano-petrous bones of Ziphius and 
Mesoplodon in my Memoir on the Cetacea of the Challenger Expedition, 
Reports, Zoology, part iv. , 1880 ; and in the same Memoir I have figured a 
tympanic bulla dredged from a depth of 2335 fathoms, which was probably 
that of Kogia. 
