Swat ELASMOBRANCHII 435 
vessels of the yolk-sac." Each embryo has its own placenta, and 
in Mustelus antarecticus the uterine portion of the oviduct is 
divided by septa into several chambers, each containing a single 
embryo.” It is worthy of note that in the viviparous species a 
distinct but very thin, delicate egg-case is formed, occasionally 
even with the rudiments of tendrils, which may either be 
retained or thrown off in the 
uterus. The Greenland Shark 
(Laemargus borealis) is unique 
amongst Klasmobranchs. Its 
egos are small and unprotected 
by egg-cases, and their fertilisa- 
tion is said to be effected in the 
water after deposition, as in the 
generality of Fishes. 
Fossil remains of Elasmo- 
branchs in the shape of fin-spines 
CGchthyodorulites) and dermal 
denticles, associated with various 
Ostracodermi (Coelolepidae, Pter- 
aspidae, and Cephalaspidae), are 
amongst the earliest undoubted 
indications of Vertebrate — life. 
They first appear in the Upper 
Ludlow Bone Bed and in Silurian Fic. 247.—Embryo of an Indian Sting 
: . eA ee ; Ray (Pteroplatea micrura) as seen 
rocks in other parts of Europe, when dhe aitoristis Iara opene ete 
and im North America; and in ‘Two bundles of trophonemata in- 
serted into the spiracles, sp, sp. 
greater or less abundance the (From Wood-Mason and Alcock.) 
group is represented in almost 
every subsequent geological period. It cannot be said that the 
group shows signs of decadence, for Elasmobranchs still survive 
in apparently undiminished numbers and variety in the marine 
fauna of the present day. 
The Elasmobranchs are certainly a very primitive race of 
Fishes. Their earliest representatives of whose structure we 
have any precise knowledge (e.g. Cladoselache and Pleuracanthus) 
are In many respects the most archaic of known gnathostomatous 
1 Leydig, Mikrosk. Anat. u. Entwick. d. Rochen u. Hate, Leipzig, 1852, p. 90 
27. J. Parker, Trans. New Zealand Instit. xxii. 1889 (1890), p. 331. 
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