STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 761 



The anterior lobes of the cerebrum have a conical form, taper- 

 ing anteriorly, and are completely separated from each other. 

 The posterior lobes, as is best shewn in side views, have a 

 semicircular form. Viewed from above they appear as rounded 

 prominences, and their dorsal surface is marked by two con- 

 spicuous furrows (Plate 38, fig. 47 B, ce'.}, which have been noticed 

 by Wilder, and are similar to those present in many Teleostei. 

 Their front ends overhang the base of the anterior cerebral 

 lobes. The basal portion of the cerebrum is an undivided lobe, 

 the anterior wall of which forms the lamina terminalis. 



What we have above described as the posterior cerebral 

 lobes have been described by Wilder as constituting the everted 

 dorsal border of the basal portion of the cerebrum. 



The portion of the cerebro-spinal canal within the cerebrum 

 presents certain primitive characters, which are in some respects 

 dissimilar to those of higher types, 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 " pro- 

 thalami." 



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 com- 

 missure is placed. We have found in the larva a large com- 

 missure in this situation (Plate 37, figs. 44 and 45, a.c.} ; and 

 it may be regarded as the homologue of the anterior commissure 

 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 37, 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 tJic 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 



B. 49 



