XVII 



ELASMOBRANCHII 



435 



vessels of the yolk-sac. 1 Each embryo has its own placenta, and 

 in Jfustelus antarcticus the uterine portion of the oviduct is 

 divided by septa into several chambers, each containing a single 

 embryo. 2 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 Elasmobranchs. Its 

 eggs 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 

 (ichthyodorulites) 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 

 rocks in other parts of Europe, 

 and in North America ; and in 

 greater or less abundance the 

 group is represented in almost 

 every subsequent geological period, 

 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. JRochen u. Haie, Leipzig, 1852, p. 90 

 ct scq. 



- T. J. Parker, Trans. Xcw Zealand Instit. xxii. 1889 (1890), p. 331. 



FIG. 247. Embryo of an Indian Sting 

 Ray (Pteroplatea micrura) as seen 

 when the uterus is laid open. t. t, 

 Two bundles of trophonemata in- 

 serted into the spiracles, sp, sj>. 

 (From Wood-Mason and Alcock.) 



It cannot be said that the 



