1886.] Central Nervous System of Plagiostomata. 11 



The structure of the peduncle resembles that of the olfactory lobe, 

 and gradually passes into that of the cerebrum at the posterior end. 

 In Scyllium, Rhina, and Acanthias it contains a passage which puts 

 the ventricle of the olfactory lobe into communication with that of "the 

 cerebrum. In Raja, however, both the lobe and the peduncle are 

 solid. 



The cerebrum contains two ventricles which posteriorly communi- 

 cate with a single chamber, the foramen of Monro ; this is the case in 

 Scyllium, Rhina, and Acanthias, but in Raja only a very small 

 ventricle is present which represents the foramen of Monro, the 

 remainder of the cerebrum being solid. Round the external surface 

 of the cerebrum there is a layer of granular neuroglia with compara- 

 tively few cells. The remainder of the parenchyma consists of a mass 

 of cells, larger ones, 13/* to \0fi in diameter, occupying the centre, 

 and smaller ones predominating towards the internal surface. In 

 Scyllium the cells are arranged in groups of four or five, and in Raja 

 also in groups of from nine to twenty-one, which make a meandering 

 pattern through the parenchyma in some parts. At the base of the 

 cerebrum there are four special groups of cells, two being placed in 

 the outer walls and two in the inner walls ; the outer groups are 

 associated with the fibres of the anterior commissure, and the inner 

 groups are associated with the fibres of the crura cerebri. 



The third ventricle is a gutter-shaped channel, long in Scyllium, 

 but shorter in Raja, which leads from the cerebrum into the optic 

 lobe ; above, it is closed in by processes of the pia mater which enter 

 the ventricle and the foramen of Monro, forming a choroid plexus ; 

 below, the third ventricle communicates by a passage, the infundi- 

 bulum, with the ventricles of the hypoarium ; the parenchyma in this 

 lobe contains numerous cells measuring from about 13^ by 7 'ju, to 6a in 

 diameter, which give off numerous processes to join a fine network 

 which pervades the Avhole. The ventricle is lined by an endothelium 

 which is continuous with a space in the hypophysis cerebri. There is 

 a small tubercle in front of the optic lobe which corresponds to the 

 tuberculum intermedium of Gottsche,* and from it a bundle of fibres 

 can be traced passing towards the ventral surface of the medulla 

 oblongata, which corresponds to the fibres of Meynerfc. 



The optic lobes which homologise with the anterior corpora 

 quadrigemina form a cover arching over the aqueduct of Sylvius, in 

 the same position as in the Teleostei ; they are much thicker, but more 

 simple in structure. Neither the tori longitudinales nor the tori 

 semicirculares, those tuberosities which form prominences on the floor 

 of the aqueduct in the Teleostei, are present in the Plagiostomata. 

 Three layers may be distinguished in the optic lobe; the external 



* " Muller's Archiv," 1835. 



