PankER.— Anatomy and Embryology of Seymnus lichia. 229 
In front of the mid-brain comes the greatly elongated thalamencephalon 
or 'bwixt-brain (the); it is best described as a shallow trough, roofed over 
only by a small band of nervous matter at its hinder end and for the rest 
by pia mater; the third ventiele or thalamocele is thus widely open above, 
and there are no lateral thickenings answering to thalami optici. 
In most Elasmobranchs,—indeed, according to Günther, in all,—there 
exist on the ventral surface of the thalamencephalon just behind the optic 
chiasma (o.c), paired ovoidal bodies, the lobi inferiores : there is no trace of 
them in Scymnus ; the thalamencephalon is merely produced ventrally into 
a thin-walled tubular infundibulum (inf), which extends backwards over the 
ventral surface of the mid-brain and is continued directly into the hollow 
thin-walled pituitary body (pty). Extending along the middle ventral line 
of the infundibulum and pituitary body is a flattened one-lobed saccus 
vasculosus (s.v). 
The prosencephalon is very interesting : instead of forming a transversely 
elongated mass, cither solid, as in Raja, or containing small ventricles, as in 
Scyllium, it consists of a small unpaired hinder portion (prc) continuous 
with and passing insensibly into the thalamencephalon, and of paired, 
divergent, pyriform bodies, the cerebral hemisphere (c. h). Similarly, the 
cavity of the prosencephalon consists of an unpaired posterior portion (pre) 
which may be conveniently called the prosocele, and is perfectly continuous 
with the third ventricle, and of paired lateral ventricles (l. v). The walls 
of the whole fore-brain are very thin, and there is no constriction between 
the lateral ventricles and the prosoccle, or between the prosoccle and the 
third ventricle. 
The olfactory lobes (o/f) are comparatively short, dilated at their ends, 
and contain olfactory ventricles (o/f.v) continuous with the lateral ven- 
tricles. 
The brain of Scymnus is thus seen to exemplify with diagrammatic 
clearness the typical structure of the vertebrate encephalon. We have the 
large fourth ventricle; the cerebellum retaining its primitive character of a 
hollow out-pushing of the roof of the fourth ventricle; the mid-ventricle 
showing no distinction into aqueduct of Sylvius and optic ventricles, and of 
approximately equal calibre with the third and fourth ventricles ; the prosen- 
cephalon, or cerebral rudiment of the embryo, composed of an unpaired 
hinder portion which bifureates in front to form the paired hemispheres, and 
these again continued insensibly into the olfactory lobes ;. the fore-ventricle 
or cavity of the fore-brain, in the form of a Y-shaped space, the stem of the 
Y being represented by the prosocwle and third ventricle, the arms by the 
lateral and olfactory ventricles ; finally all the cavities are large, and their 
walls but little thickened: this is especially noticeable in the case of the 
