BRAIN OF THE ZEUGLODONTIDiE. 



627 



oblongata is absent) was 490 c.c, or nearly 200 c.c. larger. This 

 is of some interest not only in demonstr-ating the specific (if not 

 generic) difference between these two Zeuglodonts but lilso in 

 showing that the brain-capacity of certain Zeuglodonts was con- 

 siderably greater than that of some existing Oetacea — because 

 the brain-weight of Kogia (Haswell) is only 455 grammes. 



All these distinctions between these two forms are certainly 

 not to be accounted for by a difference in the sex or age of the 

 individuals. They should therefore be separated as distinct 

 species, as has been done here, until fresh data shall be forth- 

 coming when it may be possible to associate one or the other of 

 these forms, Zeuglodon sensitivus or Zeuglodon elliotsmithii, with 

 the species now recognised from skeletal parts alone, such as 

 Zeuglodon zitteli. 



4. The three Endochanial Casts forming a Phyletic Series. 



4. a. PROZBuaLODON atrox Andrews. (M. 9265.) 



In the * Descriptive Catalogue of the Tertiary Vertebrata of 

 the Fayum of Egypb,' Dr. Andrews described briefly the out- 

 standing features of this cast, pointing out its correspondence 

 Avith Elliot Smith's general account of the Archfeoceti (1903). 

 It is unfortunate that in this important cast from the lowest bed 



Text-figure 8. 



Sagittal sinus. 



Lobus mecfius cere6e//A 



Medulla oblongata. 



AliMIe cerebral 

 vessels. 



Casserian 

 ganglion. 



-Acoustic nerye. 



Lateral view of endocranial cast of Frozeuglodon atrox Andrews. 

 About i nat. size. 



M. 9265. 



of the Middle Eocene there is no reproduction of the anterior 

 portion of the cranial cavity, because this region would supply 

 important data concerning the degree of trigeminal specialisation 

 in this early representative of the group— data which now must 

 be inferred from the evidence afforded by other regions. 



It is astonishing to find in this earliest-known Zeuglodont an 

 extravagantly expanded cerebellum. It is the dominant portion 

 of the brain from lateral, dorsal, or posterior aspects. It rises at 



Proc. Zool. Soc— 1923, No. XLI. 41 



