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MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
and have led Wilder to hold the posterior cerebral lobes, together with what we have 
called the basal portion of the cerebrum, to be structures peculiar to Fishes, for which 
he has proposed the name “ prothalami.” 
In the basal portion of the cerebrum there is an unpaired slit-shaped ventricle, the 
outer walls of which are very thick. It is provided with a floor formed of nervous 
matter, in part of which, judging from Wilder’s description, a well-marked commissure 
is placed. We have found, in the larva a large commissure in this situation (Plate 24, 
figs. 44 and 45, a.c .); and it may be regarded as the homologue of the anterior com¬ 
missure of higher types. This part of the ventricle is stated by Wilder to be without 
a roof. This appears to us highly improbable. We could not, however, determine the 
nature of the roof from our badly preserved specimens, but if present, there is no 
doubt that it is extremely thin, as indeed it is in the larva (Plate 24, fig. 46 B). In 
a dorsal direction the unpaired ventricle extends so as to separate the two posterior 
cerebral lobes. Anteriorly the ventricle is prolonged into two horns, which penetrate 
for a short distance, as the lateral ventricles, into the base of the anterior cerebral lobes. 
The front part of each anterior cerebral lobe, as well as of the whole of the posterior 
lobes, appears solid in our sections; but Wilder describes the anterior horns of the 
ventricle as being prolonged for the whole length of the anterior lobes. 
In the embryos of all Vertebrates the cerebrum is not at first divided into two lobes, 
so that the fact of the posterior part of the cerebrum in Lepidosteus and probably other 
Ganoids remaining permanently in the undivided condition does not appear to us a 
sufficient ground for giving to the lobes of this part of the cerebrum the special name 
of prothalami, as proposed by Wilder, or for regarding them as a section of the 
brain peculiar to Fishes. 
The thalamencephalon (th.) contains the usual parts, but is in some respects peculiar. 
Its lateral walls, forming the optic thalami, are thick, and are not sharply separated in 
front from the basal part of the cerebrum; between them is placed the third ventricle. 
The thalami are of considerable extent, though partially covered by the optic lobes and 
the posterior lobes of the cerebrum. They are not, however, relatively so large as in 
other Ganoid forms, more especially the Chondrostei and Polypterus. 
On the roof of the thalamencephalon is placed a large thin-walled vesicle (Plate 25, 
figs. 47 A and B, v.th.), which undoubtedly forms the most characteristic structure 
connected with this part of the brain. Owing to the wretched state of preservation 
of the specimens, we have found it impossible to determine the exact relations of this 
body to the remainder of the thalamencephalon; but it appears to be attached to the 
roof of the thalamencephalon by a narrow stalk only. It extends forwards so as to 
overlap part of the cerebrum in front, and is closely invested by a highly vascular 
layer of the pia mater. 
No mention is made by Wilder of this body ; nor is it represented in his figures or, 
in those of the other anatomists who have given drawings of the brain of Lepidosteus . 
It might at first be interpreted as a highly-developed pineal gland, but a comparison 
