REPORT ON THE SEALS. 109 



curve of the hippocampus major. Internal to it the choroid plexus was also prolonged 

 downwards, to become continuous through the great transverse fissure with the pia 

 mater covering the gyrus hippocampi. Where the cavity of the ventricle curved down- 

 wards and outwards into the horn an indication of a recess was seen in its posterior horn, 

 but it did not amount to a cornu, and there was no elevation which could be called a 

 hippocampus minor. The inner surface of the optic thalamus formed the lateral wall 

 of the third ventricle. The corpora quadrigemina were well marked, and the testes 

 overlapped the nates. 



The Pineal body or Epiphysis cerebri was remarkable for its size. In specimen c it 

 measured 30 mm. in its long and 18 mm. in its greatest transverse diameter; in b it 

 measured 29 mm. by 13 mm. It was somewhat pyriform in shape, with the apex directed 

 forwards to the optic thalami, whilst the base, which was free, projected backwards so as 

 to be visible, when the brain was looked at from above, between the two cerebral hemi- 

 spheres, where they diverged from each other posteriorly. It possessed three surfaces — 

 one was inferior, and rested in almost its whole length on that surface of the middle lobe 

 of the cerebellum which was in relation with the tentorium, and this surface was some- 

 what depressed below the level of the corresponding surface of the hemispheres of the 

 cerebellum for its lodgment. The other two surfaces were lateral, and in relation to the 

 inner and jjosterior border of the cerebral hemispheres, between which the epiphysis was 

 placed. These surfaces were slightly concave in their anterior two-thirds, so as to be 

 adapted to the convex borders of the hemispheres ; but more posteriorly, where the pineal 

 body projected between the hemispheres, they were somewhat convex, and mounted up- 

 wards to form a ridge in the inter-hemispherical interval (PI. X. fig. 1). The pineal body 

 was separated by the tentorium from the cerebrum, and was closely tied down to the 

 cerebellum by the arachnoid and the pia mater ; so close indeed was this relation, that 

 in brain a of the Walrus, which I dissected as far back as 1865, I mistook the pineal 

 body for a special thickening of the pia mater covering the middle lobe of the cerebellum. 

 The apex of the epiphysis passed forwards in front of the cerebellum and superficial to the 

 corpora quadrigernina to the region of the optic thalami, but, owing to this part of the 

 brain being somewhat friable from imperfect preservation, I could not ascertain its exact 

 connections, though there can, I think, be little doubt that, as in other Mammals, it 

 was attached to the thalami by a pair of peduncles. 



The very remarkable size of the epiphysis cerebri in the brain of the Walrus, and its 

 unusual development also in the brain of the Seals, are of especial interest in connection 

 with recent important observations on the connections and homology of the pineal body 

 Elders showed ' that in the Plagiostomata the epiphysis cerebri is lodged in a depression 

 in the cartilaginous cranium, whilst retaining its connection with the brain through its 



1 Zcitschr.f. u-iss. Zool., Bel. xxx., Supplement, 1878. 



