BRAIN OF THE ZEUGLODONTID.E. 623 



tlie enlarged lolnis floccularis on either side and in tlie'mid-rogion 

 tlie so-called lobus medius. The lobus medius of the cerebellum 

 IS shown to be markedly asymmetrical. A bilaterally symmetrical 

 projection into the posterior part of the tentorial median depres- 

 sion possibly represents the lobulus simplex of Bolk. If so, it is 

 very small, and the lobus anticus is completely hidden from sight 

 underneath it. 



The hemming in laterally and anteriorly of the hemispheres at 

 their ventral margins by the Gasserian ganglia is obvious from 

 this aspect— forming a unique arrangement for a mammal. In 

 no other mammal have these ganglia been described as visible 

 from the dorsal aspect of the brain. The ganglia therefore are 

 peculiar as constituting a factor in the modelling of the roof of 

 the cranial cavity. 



The ventral surface of the natural cast (text-fig. 3) gives im- 

 portant confirmation of the inferences drawn already. Through 

 this cast we are able to infer the extent and size of the huge 

 Gasserian ganglia. With these must be related a correspondingly 

 extensive area of grey matter in the medulla oblongata and coid 

 (substantia gelalinosa Uolandi). The rcaomblanco of this ns])ect 

 of the casts to the ventral surface of the brain of Ornithorhynchus 

 (which is the only mammal that provides a suitable comparison, 

 by virtue of its similar functional specialisation) is undoubtedly 

 the clue in this arrangement. Anteriorly', the ophthalmic trunk of 

 the trigeminus overlaps the optic nerves and olfactory peduncles. 

 The course of all three structures is pivrallel for some distance 

 until the regioi^ of the tuberculum olfactorium is reached. The 

 maxillary division of the trigeminus is there attached to the 

 ganglion^ Between the ganglion and the tuberculutn the optic 

 nerve (not visible as a distinct elevation on the surface of the 

 cast) must have skirted the lateral aspect of the tuberculum till 

 it reached its posterior margin, where it bent medially to meet 

 its fellow of the opposite side in the optic chiasma, the position of 

 which can be recognised at the posterior end' of an "inter-tuber- 

 cular sulcus" lying in the mid-line between the two tubercula. 

 Directly posterior to this point there is a single median elevation 

 present in the three casts. It is obviously the site of the 

 strongly-marked tuber cinereum and hypophysis. Behind this 

 region the Gasserian ganglia diverge, giving place first of all to a 

 slight central depression, then to a broad flat surface. This wide 

 divergence and a bulging of the cast on either side of the mid-line 

 may be due in large measure to the forward jutting of a tuberculum 

 quinti upon either side correlated in size with the enormous 

 Gasserian ganglia. In the cast we have no indication that an 

 elevation due to a pons Varolii, as such, was present, and probably 

 it possessed no lai'ger pons than the Prototherian. The " bulging 

 of the cast" postero-medial to the Gasserian ganglion may there- 

 fore be interpreted as the upper portion of the tuberculum quinti. 

 Behind it lies a transversely running depression in the region 

 where the pons would normally lie. It is possible that this 



