BRAIN OP THE ZEUGLODONTID^. 635 



with those of the foregoing which are ahuost twice the size. 

 An examination of the tables and the diagrams of the skulls will 

 show that this discrepancy in brain -capacity is not due to any 

 diminished bodily size in Zeioglodon osiris, for although the skull 

 may be somewhat shorter it is absolutely wider than those of the 

 other two forms. 



One can only conclude that we have here an obvious dwindling 

 in brain-substance which has affected not only the trigeminal 

 ,'ind olfactory regions but with them the cerebrum, cerebellum, 

 and the medulla oblongata. The whole evidence goes to show 

 that the devolutional potentialities exhibited in the specialisations 

 (chiefly trigeminal) of Prozewjlodon and the group it typifies have 

 found in Zeuglodon osiris (M. 10228) their logical finale. 



6. The ZooLOGicAii Position of the Zeuglodontid^. 



Certain facts arising from our study of these endocranial casts 

 claim our immediate attention. The first of these is the tri- 

 geminal specialisation. ; In all of the Zeuglodonts (although 

 coming from different horizons) the essential features noted in 

 Zeuglodon sensitivxis apply. In all, the Gasserian ganglion assists 

 in modelling the roof of the cranial cavity. The inference is 

 justified that the group, as a whole, rested from the outset upon 

 this specialisation for its subsequent achievements. 



It seems to be a law of general evolutionary application that 

 specialisation of one *' sense " entails as its corollary the atrophy 

 of one or more of the other avenues of sense-perception. Thus, 

 birds become specialised as to sight but lose their appreciation of 

 smell ; the same is true, though in different ways, for Teleostean 

 fishes and for Primates. In Ornithorhynchus, which specialises 

 in its " fifth nerve sense," we find a relative atrophy of both smell 

 and sight. In Zeuglodontidro the sense of smell has certainly 

 been largely lost as a result of their adoption of a water habitat, 

 a,nd it seems likely that sight also was of diminished value. 



To the loss of smell and the abortion of the basal (olfactory) parts 

 of the cerebrum with their effect in limiting the longitudinal exten- 

 sion of the hemispheres and consequently of the cranial cavity, 

 reference was made by Elliot Smith (1902 and 1903). A I'elative 

 loss of sight, with a resulting diminution of the mid-brain and 

 thalamic regions, may assist in accounting for the smallness of the 

 cerebrum in Prozeuglodon and also for its lack of growth in the 

 two later forms from the upper horizons. It assists in the under- 

 standing of the propinquity of the cerebellum to — or, rather, its 

 overgrowth over — the cerebrum, and affords further reason for 

 the diminished longitudinal extension of the cerebral axis (because 

 of a wasting mesencephalon and optic thalamus and the con- 

 sequences entailed thereby) throughout the series. 



13 ut it seems to me that the lateral expansion not merely of 

 the brain-stem itself but also of the cerebellum has been provoked 

 mainly by the trigeminus, and that a specialisation of this nerve 

 has been the most significant factor in determining the queer 



