PROSENCEPHALON OF SPINAX AS A TYPE OF ELASMOBRANCH FORE-BRAIN. 495 
medial olfactory nucleus, cross to the opposite side and enter the. primordium 
hippocampi. A similar decussation is seen in my preparations of the frog brain, 
and has been described by Snessarew.” In his figures this tract forms a well- 
defined cross, but in Spinax there exists only the slightest indication of these 
fibres, and one is not certain that they are the same (fig. 8). 
The following two tracts lead to the thalamencephalon : — 
The tractus olfacto-hypothalamicus.- — Figs. 7 and 8 show part of the dis- 
tribution of the lateral portion of this tract. Its course corresponds, allowing for 
the difference regarding the homologies of the neurone areas of the brain, with 
that described by Johnston. There is a lateral and a medial portion, which 
form a diffuse bundle lying in front of and terminating posteriorly in the 
anterior commissure and medial fore-brain bundles (figs. 9, ll). The anterior 
commissure is apparently largely composed of decussating fibres from the medial 
fore-brain bundles. 
Tractus taeniae of Edinger, Kappers ; tractus olfacto-habenularis of Johnston. — 
Part of this tract is seen in fig. 11, and its course apparently agrees with that 
described by Johnston. It consists of a number of fibres, among those of the lateral 
olfactory tract, which gradually come into view as one follows a series of sections 
posteriorly, becoming more and more definite. 
The following five tracts are connected either with the primordium hippocampi or 
with the formatio pallialis : — 
Tractus pallii (fig. 10) represents part of this tract. It was firstly described by 
Edinger, and has also been noted by later writers on the Selachian fore-brain. 
It does not require further description, but I may note that it appears to be less 
developed in Spinax than is the case in some related forms. 
Tractus cortico-habenularis (Johnston) ; tractus olfacto-habenularis (Kappers). — 
This tract is very small, but I have observed a few fibres dorsally behind the inter- 
ventricular foramen, which are apparently joined by several fibres proceeding from 
the primordial hippocampus. They are not very clear, but the probability is that 
these constitute the above tract, which is apparently more developed in Scyllium than 
in Spinax and Acanthias. 
Commissura hippocampi (fig. 10).— 1 This commissure, situated in the roof of the 
fore-brain, has been described by various authors. It has been termed the upper part 
of the anterior commissure, the pallial commissure, and the psalterium. This tract 
is not so well developed in Spinax as in Acanthias. Johnston writes (p. 19) : “It 
is certain, however, that there are fibres which arise in the primordium hippocampi 
itself and cross to the opposite side. In Scyllium and Raja these fibres are diffusely 
scattered among the olfactory decussations, but in Acanthias part of them at least 
form a distinct bundle whose course is worth describing.” In Spinax I have not 
been able indubitably to detect fibres crossing to the opposite side, but as apparently 
only some of these fibres are medullated, it is possible they may have escaped my 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 18). 75 
