es eae 
B. G. Wilder—Anterior brain-mass with Sharks and Skates. 105 
furrow or fissure. D. In Odontaspis and some others there are 
posterior prolongations of the ventricle upon a higher plane 
than the anterior horns of the original cavit 
uller series of foetal brains is required for absolute 
demonstration, but the seeder above described suggest the 
following conclusion : The hinder part of the mass corresponds 
to the prothalami of Ganoids. The true hemispheres are the 
lateral masses more or less completely fused on the middle line 
(as are the eee lobes of frogs and_toads,) and sometimes 
to be formed ine th de It and probable therefore that the 
commonly accepted definitions of the hemispheres and the 
olfactor ry lobes must be modified with reference to the condi- 
ons here indicated: —the latter being developed first and 
directly from the ad cerebral vesicle; the former to the 
nner or mesial side the 
olfactory lobes. Their rel- 
reverse of what they are in 
Petromyzon, notwithstanding 
the other points of resem- 
EXPLANATION OF FIGURES. 
at I—Diagrams of sections of the large median anterior brain-mass of 
= tks. A. From embryo Mustelus 37 mm. long. Horizontal perce (enlarged) 
the thin-walled mass a seems to be the slightly modified front and 
ve From foetal Acanthias 16 em. long. Horizontal secti 
n, enlarged. 1 te 
re thicker ; th en rotrusions of © 
ib twins tarminatia e itactry lobes are larger, an and the Pp 
e. 
th From an adult paar The olfactory lobe has either expan ded in 
teary connected with a . nee crescentic bulb into which the ventricle does ne! 
aps e lateral protrusions (H) are closely united by their mesial surfaces 
the Loe). The foramen mild — the position of the median line of 
oe terminalis. The much reduced by the thickening of the 
