1884.] NATURAL SCIENCES OF PHILADELPHIA. 269 



as the commissure figured by Professor Balfour ^ in the Elasmo- 

 branch brain. It passes across the posterior ends of the hook-like 

 processes of the thalami, which I have compared with the habenulae, 

 and the most satisfactory interpretation of this commissure is 

 afforded by a comparison with Ahlborn's figures of the lamprey 

 brain. 2 At the sides and to the front of the recessus pinealis^ I 

 find in Menopoma two compact masses of nerve cells, which I 

 think we may compare with the ganglia habenarum. These 

 masses form the posterior, and to some extent the inferior, 

 boundary of the supracommissura. Following the fibres of this 

 commissure downwards and forwards, we find that they partly 

 enter the thalami, while the greater part pass directly into the 

 hemispheres. Their distribution, then, is similar to that of the 

 fibres of the teenia thalami optici, while the commissural portion 

 may be compared with a slender commissure, the commissura ten- 

 uissima^ traversing the habenulae in the lamprey's brain. The 

 relations to the hemispheres are especially interesting, as they 

 indicate, between the posterior parts of these bodies, a commis- 

 sural union of considerable extent and importance. 



Infundibular Commissures. — The lobes of the infundibulum 

 are united dorsally and ventrally by two commissures, the 

 uppermost being quite distinct and extensive (fig. 4) and forming 

 the thin fold which divides the iter from the infundibular cavity. 



The Hypophysis and Epiphysis. 



The backward extension of the hypophysis, together with its 

 great development, and the unusual size of the infundibular cavity 

 and lateral lobes, lend this portion of the brain especial interest. 

 I will, however, only remark here upon the clear separation of 

 the anterior and posterior lobes of the hypophysis, by the turning 

 in of the pia mater over the forward face of the anterior lobe (fig. 

 4). The vessels of the pia ramify between the columnar epithe- 

 lial cells, which compose the tubes forming this lobe. In vertical 

 section the lumen of one of these tubes is occasionally seen. The 

 ependj'ma is much convoluted in the posterior lobe, and these 

 foldings may readily be mistaken for tubes. 



Our knowledge of the epiphysis in the Amphibia is in a far from 

 satisfactory state. There can be little doubt as to the correctness 



* Elasmobranch Fishes, plate xv. * Loc. cit., p. 285. 



