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Show 944 DR. W. G. RIDEWOOD ON THE [NoV. 28, condition is more primitive than that found in Megalops (fig. 4), which the third efferent vessel unites with the fourth on each side to form a short common trunk which carries the double charge of blood to the aorta. If this line of argumentation be extended to the cases in which the third and fourth vessels open into the circulus cephalicus instead of into the aorta, we are led to the conclusion that Blennius (fig. 35), having the Y-shaped system, is more specialized than Gadus (fig. 34), where the third and fourth vessels are disposed in the form of a V, and that this latter is more specialized than Syngnathus (fig. 33), where the two vessels find separate outlets into the circulus. Whether much importance, however, can be attached to this last feature is open to considerable question, for the V-shaped system obtains in Salmo (fig. 7) and the Y-shaped one in Osmerus (fio*. 10), and a similar relation exists between the Chinese Carp Hypophthalmichthys and our native Carp Cyprinus (fig. 13), while among the Siluroids, a presumably natural assemblage of forms, there are gradations from Callichihys (fig. 19) and Liocassis (fig. 18) with the V-shaped system, through Clarias (fig. 20), and Silurus (fig. 32), to Saccobranchus (fig. 31), with a typical Y-shaped arrangement. The disposition of the efferent branchial vessels is independent of the shape of the head, except in so far as the slope of the vessels and the shape of the circulus is concerned. There is no connection, that is to say, between the shape of the head and the degree of suppression of the median aorta, or the entry of the efferent branchial vessels into the circulus rather than into the aorta. In long-headed forms like Ammodytes (fig. 3), Sphyrcena, Fistularia (fig. 30), Anguilla (fig. 16), and Syngnathus (fig. 33) the circulus cephalicus is elongated in an antero-posterior direction; while in Cottus (fig. 27), Lophius (fig. 26), and others, with a broad, flat head, the gills are widely separated, and the circulus cephalicus is proportionately broad, the common trunks formed by the fusion of the third and fourth efferent vessels being also lengthened. The differences in the arrangement of the efferent branchial vessels relatively to the circulus cephalicus and the aorta are not correlated with any differences in the position and extent of development of the epipharyngeal dentition. At the commencement of the inquiry, the suggestion occurred to m y mind that the development of a large ancl elaborate dental apparatus might, by some process of natural selection, have resulted in the blood-vessels taking up a position of safety, out of the line of direct pressure between the epipharyngeal bones and the vertebral centra or the base of the skull. A minute examination of the individual cases shows, however, that the vessels do not experience any displacement under the circumstances, but obtain sufficient protection by running in grooves or arches in the epipharyngeal bones ; in fact, as often as not, the epipharyngeal teeth lie immediately below certain of the efferent branchial blood-vessels. As an instance of two forms |