150 Journal of the Asiatic Society of Bengal. [N.S., XV, 
sharks of the genus Scoliodon. We have thus reached a stage 
where we find the formation of a definite placental connection 
between the embryo and the mother for the absorption of food. 
A still more interesting modification is to be seen in the aplacental 
viviparous Elasmobranchs which absorb, or injest, the nutritive 
secretions of the uterine glands through special structures de- 
veloped either by the embryo or the maternal uterus or by both. 
In these forms though there is no definite placental connection, 
still the nutritive material from the uterus reaches the embryo 
in an indirect fashion. 
s to the origin of the placental and aplacental viviparous 
forms our views are entirely in agreement with those of Alcock, 
and may be summed up as follows :—Placental viviparity 
branchs the evolution of the viviparous from the oviparous 
forms has taken place along two definite lines—(i) placental, (ii) 
aplacental. 
In passing, one cannot help remarking on the existence of 
the various types of oviparous, placental and aplacental vivi- 
parous forms amongst the Elasmobranch fishes. 
e will now go on to give a short general account of some 
of the accessory structures developed by the viviparous forms. 
_ __ Placenta.—It should be clearly understood that the placenta 
is of the true yolk-sac type, and that in these forms we can trace 
ries in the evolution of the arborescent, spreading 
type of placenta of the more advanced forms from the simple 
