PROSENCEPHALON OF SPINAX AS A TYPE OF ELASMOBRANCH FORE-BRAIN. 493 
hippocampi are situated dorsal to the formatio pallialis. Ventrally, the tuberculum 
olfactorium forms the bed in which the medial fore-brain bundle is later situated, as 
seen in text-fig. 2, which illustrates the arrangement slightly posterior to the last. 
The tuberculum olfactorium is no longer so obvious, and the position formerly 
oecupied by the corpus paraterminale and the corpus striatum is now represented by 
a transverse area which has been termed the thalamic gray s Text-fig. 3 illustrates 
the arrangement of the parts further posteriorly in the thalamencephalon at the 
region from which the optic nerves originate. The areas have naturally undergone 
further re-arrangement, and the medial fore-brain bundle is much more pronounced. 
Part II.— THE FIBRE TRACTS. 
The following account is mainly concerned with the course of the medullated 
fibres in the prosencephalon, dealing more especially with those which apparently 
vary from the results attained by other authors. 
Tractus olfactorius. — The course of this tract in Spinax mainly agrees with that 
described by Johnston in Scyllium and Acanthias, after allowing for the divergence 
in the method of viewing the neurone areas of the fore-brain. 
The following four tracts appear to be confined to the prosencephalon 
proper : — 
Tractus olfacto-corticalis lateralis. — A few medullated fibres are seen on the 
lateral side of the dorsal end of the lateral ventricles (fig. 5). These fibres have 
an oblique course, and apparently run from the formatio pallialis to the primordium 
hippocampi. Johnston has described a similar set of fibres as the tractus olfacto- 
corticalis lateralis rectus and the tractus olfacto-corticalis cruciatus in Scyllium and 
Acanthias. He states that some of these fibres enter the large roof commissure 
and pass to the opposite half of the primordium hippocampi ; but in the fore-brain 
of Spinax I have been unable to detect such fibres crossing over to the opposite 
side. The fibres of the tractus olfacto-corticalis lateralis are firstly seen far 
anteriorly, when the peduncular part of the fore-brain is still evident, and when the 
small anterior part of the lateral ventricle is seen for the first time. These fibres 
are situated lateral and dorsal to the apex of the ventricle, and are sometimes 
not clearly separated from the fibres of the tractus olfactorius. 
Tractus olfacto-corticalis medio-dorsalis ( fasciculus marginalis of Elliot Smith 
in Ornithorhynchus). — Fig. 6 shows part of the distribution of the fibres of this 
tract. The fibres are seen in the anterior part of the prosencephalon, when the 
lateral lobes are fused in the median line for a considerable extent. They run 
dorsally and medially from the zona limitans medialis, and then parallel with the 
dorsal margin of the prosencephalon until they meet the fibres of the tractus 
olfactorius lateralis, hence the term fasciculus marginalis appears appropriate. 
