BY G. ELLIOT SMITH. 637 



indented to profluce two parallel furrows, in exactly the saijie way 

 as occurs among the Eutheria. The lower of these two furrows, 

 which extends forwards as far as the foramen of Monro and 

 backwards as far as the posterior extremity of the lateral ventricle, 

 is the Jissura choroidea. The upper corresponds to what has been 

 called the Jissura arcuata ( Bogenfurche of Arnold) in the placental 

 mammal and is coextensive with the lateral ventricle. Included 

 between these two fissures is a strip of cortex, which is known 

 as the arcus marginalis { RandbogeaJ, which (owing to the 

 deficiency of the cortex lining the fissura choroidea) becomes the 

 edge of the cortex cerebri. The fissura arcuata of the Monotreme 

 and Marsupial brain differs from the corresponding structure in 

 the placental mammal, in that it persists into adult life as the 

 fissura hippocampi in the whole extent of the ventricle, thereby 

 meriting the name of " Avimonsfurche" given to the corresponding 

 structure in the Eutherian brain by Mihalkovics. In the non- 

 placental mammal, then, the hippocampal fissure forms a projection 

 — the hippocampus — into the lateral ventricle in its whole extent. 

 The fissure, and consequently the hippocampus, corresponds in 

 shape with the contour of the ventricle. So that in Platypus, 

 where the lateral ventricle has only a small descending horn (so 

 small that it was quite overlooked by Hill, " On the Cerebrum of 

 Ornithorhynchus," (Philosophical Transactions B. 1893), the 

 hippocampus and hippocampal fissure present a slight hook-like 

 bend downwards at their posterior extremity. This is shown in 

 a somewhat exaggerated manner in fig. 2 hf. In all Marsupials 

 the fissura hippocampi (fig. 1 hf.) presents a well marked curve 

 downwards at the posterior extremity corresponding to the down- 

 ward curve of the lateral ventiicle to form its descending horn. 

 From the arcus marginalis is formed the fascia dentata and from 

 the projection into the ventricle the cornu Ammonis, whose 

 histological structure closely resembles that of the corresponding 

 regions of the Eutherian brain, as described by Ramon y Cajal.* 



* " Neue DarsteUung vom histolog. Ban des Central-nervensystems, 

 Archiv fiir Anat. p. 377, 1S93. 



