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Discussion.
The general characters of the vascular system by which the rays are held to differ from the sharks are as follows:—The lateral dorsal aortae are interrupted; the first two epibranchial arteries do not enter the dorsal aorta separately; the three posterior afferent arteries leave the ventral aorta by a common stem; the lateral hypobranchial arteries are ventral to the afferent arteries; there are no median hypobranchial arteries; the anterior mesenteric artery supplies the dorsal side of the intestine only; there is no ventral intestinal artery; there is a single posterior cardinal sinus; the lateral abdominal veins are not united posteriorly by a pelvic vein but arise from small branches in the cloacal region.
This type of vascular system is found in R. nasuta, which is typical of the Rajiformes. The Torpedonidae are also very similar, with the addition of arteries and veins supplying the electric organ. Typhlonarke is one of the most highly specialised members of this family in that its eyes are rudimentary, the fins are reduced in size and the bases of the pectoral fins are very far back. It has no orbital
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artery but otherwise its vascular system is essentially similar to that of Torpedo and Raja. It is interesting to find in the Stingray, however, certain shark-like features. The first two epibranchial arteries join the dorsal aorta separately, the three posterior afferent leave the ventral aorta separately, and there is a pelvic vein connecting the posterior ends of the lateral abdominal veins. Though it could not be determined for certain in the single specimen available it seemed that the posterior cardinal sinuses do not fuse into a large median sinus. It is impossible to generalise upon details of the vascular system of two species only, but it seems possible that an extensive study might produce evidence that the Centrobatoidei represent a different line in the Rajiformes as compared with the Rhinoraji and Torpedinoidei.