Parxer.— Anatomy and Embryology of Seymnus lichia. 227 
and slightly upwards, parallel with the cutaneous ridge described above, to 
the pectoral fins, where it is connected with the brachial veins. It thus 
marks exactly the position of Balfour's lateral ridge, or in other words of 
the hypothetical ancestral lateral fin. It is worthy of notice that the body- 
muscles are disposed peculiarly with regard to this vein (fig. 4), a transverse 
section showing that the muscular bundles are disposed around it in a radi- 
ating fashion. The section also shows that the ridge in question is not a 
mere cutaneous structure like the lateral keels on the tail of Lamna, Car- 
charodon, etc., which are formed merely by a thickening of the tough, 
white, fibrous tissue of the dermis. In the pre-pelvic ridge of Scymnus, on 
the other hand, the skin is no thicker than in other parts, but is moulded 
on an actual muscular ridge. Throughout the greater part of its course 
the lateral vein lies immediately beneath the peritoneum. 
Physiologically, I am disposed to think that the lateral vein has but 
little significance, since except at its anterior and posterior ends it receives 
only the small veins from the abdominal walls. This, coupled with the 
structural peculiarities just mentioned, seems to confirm the view I advanced 
in describing the corresponding veins in the skate, namely, that the lateral 
vein represents the vein of the primitive vertebrate lateral fin. It seems 
possible also that it may be genetically derived from the lateral vessel of a 
more remote vermian ancestor, but this is merely a suggestion. 
The lateral vein exists also in Acanthias vulgaris, Mustelus antarcticus, 
and Chiloscyllium furvum, in which, as in Scymnus, it is so obvious a struc- 
ture that, in spite of the absence of any mention of it in the books at my 
disposal, I feel sure it must have been previously noticed. 
4, Urinoyenital organs. 
The kidneys are very long, extending nearly to the anterior boundary 
of the body-cavity, and apparently representing both meso- and meta- 
nephros. That the mesonephros should remain functional in the adult female 
is noteworthy, since from the analogy of other Selachians it is probably con- 
verted in the male into the epidymis. A single ureter runs alongside the 
inner edge of each kidney, widening posteriorly, and finally dilating into the 
urinary bladder. Projecting into the cloaca at its anterior end is an unusually 
large median urinary papilla (fig. 1, u.p), on the ventral surface of which, 
near the apex, is the single urinary aperture; this leads into a compara- 
tively narrow canal in the very thick-walled papilla, and into the anterior 
end of the urethral passage thus constituted the two urinary bladders open. 
The oviduets open into the cloaca by widish apertures (fig. 1, ut’), one 
on either side of the urinary papilla. The posterior part of each (wt) is 
wide, having in the gravid state a considerably greater diameter than either 
the stomach or intestine, and forming a uterus or brood-pouch: a little 
