52 Journal of Comparative Neurology. 



a granular fibrous structure and in its outer portion contains the 

 thick fibres of the optic nerve. Beneath this in cross section 

 there appears a concentric fibre layer. Throughout the entire 

 layer are scattered circular elements. Besides the concentric 

 striation there is a radiating system of fibres a part of which are 

 formed by the processes of the deeper cell- layer, while a much 

 larger portion is apparently derived from n])tic tract fibres which 

 turn ventrad. The minute relations of these fibres were not 

 investigated. The second layer is formed of two or three 

 rows of granular cells (Kornerzellen). These cells send their 

 processes perepherad. Between the series of cells of this layer 

 is a system of longitudinal fibres not further determined. The 

 third layn- consists of the epithelium of the ventricle. Mesad the 

 two halves of the tectum are connected by a transverse commis- 

 sural system forming the continuation of the concentric fiber 

 system of the external layer of the tectum. Between the fibres 

 of the commissure are a vast number of granular cells. The 

 commissure is more strongly developed cephalad where it passes 

 into the commissura posterior. In the cephalo-mesal part of the 

 tectum are a number of large rather closely set ganglion cells 

 whose fine processes pass radially toward the surface of the 

 tectum, where they seem to pass into connection with the optic 

 fibres. These cells are obviously the large ganglion cells desribed 

 by Rohon under the name Dachkerne in Selachii, in which group 

 they are restricted to the middle of the tectum and extend farther 

 caudad than in the sturgeon where they are limited to the anterior 

 portion. 



Where the optic tracts ])ass into the tectum there is no 

 division into anterior and posterior bundles. 



VIII. — The nidulus genie ulaius externus, which appears on 

 the surface of the brain as an obvious protuberance or " corpus 

 geniculatum." is always present but not always obvious. On 

 plate XVII, Vol. I, Fig. 4, this projection appears as a small 

 tubercle adjacent to the optic tracts and cephalad of the hypo- 

 aria. It was described by my brother, in the paper alluded to, 

 its relations are more clearly seen in the cross-section, Fig. 8. 

 We recall no other instance where it is so distinctly obvious 

 upon the surface as in the cat fishes. It is bounded cephalad by 

 the optic tracts and tract of the transverse commissure, is trav- 



