NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 25 



Platanistidae. — The Platanistids no doubt originated from the 

 most typically defined Squalodonts such as Squalodon. The most 

 important and perhaps in the first place the only distinction between 

 the Platanistids and their precursors among the Squalodonts is that 

 the teeth in the Platanistids are to a higher degree structurally degen- 

 erated. They have lost their heterodonty, have become smaller but 

 more numerous, all of them nearly simply conical with a single root. 

 On the other hand the Platanistids have retained most of the other 

 peculiarities in which the Squalodonts show themselves to be relatively 

 primitive. Especially noticeable in comparison with higher cetacea 

 are the following characters : facial depression rather narrow, not 

 much widened laterally behind, so that its outer margin covers over 

 the temporal fossa to a slight degree only ; temporal fossa rather 

 large ; zygomatic process of the squamosal robust ; all these peculiari- 

 ties are no longer or scarcely ever found among cetacea of the higher 

 families. The braincase appears to be rather small and not very 

 much compressed antero-posteriorly, this also in contrast with the 

 higher whales. In common with the most primitive forms of the 

 higher cetacean families the Platanistids have, so far as they are 

 known, such peculiarities : as the mutual independence and rather 

 considerable size of the cervical vertebrae ; as the conspicuous lack of 

 uniformity in the shape of the dorsal vertebrae (for instance the 

 long, broad transverse processes of the lumbar vertebrae in contrast 

 with the rather short processes of most of the thoracic vertebrae ; only 

 on a few of the hindmost thoracic vertebrae do there occur robust 

 transverse processes, parapophyses, which bear ribs at their tips in 

 the Platanistids that are known in this respect) ; as the well-developed 

 heads to the anterior ribs, and probably also the coalescence, or more 

 correctly the non-separation, of the tuberculum and capitulum on the 

 hinder ribs ; as the rather large, ossified costal cartilages ; as the rather 

 well-developed sternum ; as the presence of the first finger, the meta- 

 carpal at least of which is found ; as the rather slight lengthening 

 of the middle fingers, etc. 



In one single direction the known Platanistids have developed 

 themselves highly. They have used the jaws as a kind of delicate 

 forceps to seize and hold prey that did not make very strong resis- 

 tance. The jaws grow to an unusual length but become noticeably 

 slender, fine, though solid. The intermaxillary and maxillary press 

 close together, covering over the anterior part of the mesethmoid, and 

 they have a tendency to coalesce. The maxillary has pushed itself 

 forward anteriorly beyond the tip of the intermaxillary. In the lower 



