152 ROY L. MOODIE 
acusticum’ is a structure (figs. 5, 6, 8, 15) which at first was 
provisionally identified as the pineal body and its stalk. In all 
modern forms, however, the epiphysial stalk is directed upward 
and forward from the diencephalon, not backward, as in these 
specimens. In some living elasmobranchs there is a dorso- 
medial vein in approximately this position,” though nothing has 
been described which resembles this structure exactly. In the 
specimen of the Mississippian Rhadinichthys, the posterior part 
of the midbrain roof has been shattered; but at the anterior end 
there are two small ridges (fig. 4) which resemble the dorsal 
veins in some of the species of elasmobranchs figured in the work 
of Rex cited above. It is probable that these ridges in all of 
our specimens are dorsal veins, and that the enlargement marked 
* in figure 15 is, in reality, a venous sinus. 
The structure labeled ‘thalamus’ (figs. 4, 5, 6, 9, 17) is situated 
anteriorly between the optic lobes and the cerebral hemispheres 
(olfactory lobes). It is single and median, with its upper surface 
partially abraded in the specimen figured. The full form is as- 
certained from a study of its mold in other nodules. Posteri- 
orly and between the thalamus and the optic lobe there may 
be seen a small structure designated on figure 17 ‘oculomotor 
nerve;’ this may, however, be a blood vessel. Its peripheral re- 
lations cannot be determined. Anterior to it and lying in the 
lower part of the thalamus are four small but distinct elevations, 
which doubtless represent nuclei of uncertain function. 
The hypothalamus (lobus inferior) is very large, as in most 
recent fishes. It occupies the typical position below and behind 
the thalamus and below the optic lobes. An eminence at its 
caudo-ventral angle is probably the hypophysis. 
The cerebral hemispheres (marked ‘olfactory lobes’ on the 
figures) are small as compared with those of living fishes, al- 
though they are distinctly ganoidean in form. The region in 
front of the expanded olfactory lobes tapers forward slightly 
into a hollow stalk (figs. 7 and 15), whose cavity is the rhino- 
coele. If the olfactory bulbs lie farther forward, as in many 
22 Rex, H. Beitriige zur Morphologie der Hirnvenen der Elasmobranchier. 
Morph. Jahrb., Bd. 17. 1891, p. 417. 
