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Show 942 DR. W. G. RIDEWOOD ON THE [Nov. 28, and the consequent difficulty of dissection, or to the bad state of preservation of the parts, it was not possible to trace out the vessels so delineated, but that, from analogy with other forms, there is reason to believe that the vessels occupy the positions indicated. There appears to be no reason to suppose that the circulus cephalicus is ever incomplete in front. The lumen of the transverse commissure between the roots of the anterior carotid arteries may possibly be closed in some cases, but the failure of the injection-mass to pass into the commissure is not necessarily a proof of the fact, since the pressure during the process of injection is equal on the two sides of the circulus. The transverse vessel is usually of small size, and its traversing the parasphenoid bone makes it difficult to dissect out with any degree of neatness. In the selection of characters by which to classify the various types of arterial disposition, it has been assumed that the condition found in Clupea (Pl. LXIII. fig. 2) and Engraulis (fig. 1), where the circulus cephalicus is small, and does not involve the second, third, and fourth efferent branchial vessels, is the most simple and primitive, and that the connection of all four efferent branchial arteries with the circulus, such as occurs in Gadus (Pl. L X V . fig. 34), is the most specialized. This assumption is based partly upon the fact that the Cadoids are highly specialized in numerous other respects, whereas the Clupeoids are generally recognized as among the lowest of the Teleostean series; partly upon the fact that in Amia, an admittedly primitive Ganoid with Clupeoid affinities, the last three efferent branchial vessels are unconnected with the circulus cephalicus l; partly also upon the researches of Ayers (4) upon Elasmobranch fishes, which go to prove that the right and left sides of the circulus cephalicus are not the primitive paired aorta? such as occur in Amphioxus and in embryos of the true Vertebrata, but that the true dorsal aorta may persist as a median vestigial vessel traversing the circulus cephalicus, iu the same manner as, according to Muller (16), it does in the Cvclostomi. Pursuing this line of argument, we may legitimately conclude that where, as in the Salmon (fig. 7), Mackerel (fig. 6), and Carp (fig. 13), the circulus cephalicus, receiving the first and second efferent branchial vessels, is separated from the point of entry of the third and fourth by a length of the median aorta, the condition is more primitive than that in which the third and fourth vessels open at the posterior extremity of the circulus cephalicus, as in the Bass (Pl. LXIV. fig. 17). And further, the separation of the third and fourth vessels in the Anchovy (fig. 1) by a portion of the aorta indicates a more lowly condition than that seen in the Herring (fig. 2), where the two vessels open close together. There are thus two lines upon which we may regard specialization as proceeding: firstly, by the circulus cephalicus 1 If such exists. The circulus appears to be suggested in Allis's figure (3. pl. xxxvi.), but its existence is denied by Ramsay Wright (25. p. 495). The arrangement in Lepidosteus is somewhat similar to that of Amia See Hvrtl 8 p. 235, and Muller, 17. pl. v. fig. b\ |
