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Show 952 DR. W . G. RIDEWOOD O N THE [NOV. 28, branchial vessels into the circulus, aud the more posterior of the subclavian arteries. In Fistularia (fig. 30) the slope of the common trunks of the third and fourth efferent branchial vessels is so slight that the genus might with equal propriety be classed under subgroup C b, and the fact that these common trunks do not enter exactly at tbe posterior angle of the circulus cephalicus makes it difficult to uphold its claim to come into group C at all. The circulus is long and abruptly terminated in front. The aorta, after giving origin to tbe coeliaco-mesenteric artery, is unsymmetrical, and runs to the left side of the vertebral centra. In Siiurus (fig. 32) the posterior angle of the circulus cephalicus is not exactly coincident with the mesial ends of the common trunks of the efferent branchial vessels 3 and 4, as it is in Saccobranchus (fig. 31); and thus the form really occupies an intermediate position between the subgroups Cc and Be, in which latter subgroup the Siluroid genus Mulapterurus (fig. 12) has already been placed. AVith regard to Saccobranchus, it has already been pointed out by Hyrtl (10. p. 306) that the first branchial vessel unites with the second (which is another way of stating that they both open into the circulus), the third with the fourth, and that the efferent vessels of the lung-sac open into the fourth branchial vessel. FURTHER REMARKS ON GROUP D. In Syngnathus (fig. 33) the dorsal aorta is not median but runs slightly to the left side of the vertebral centra. The coeliaco-mesenteric artery arises at the posterior angle of the circulus cephalicus. From the same place arises a single vessel forking posteriorly into the two subclavian arteries. Hippocampus closely resembles Syngnathus, but since the openings of the third and fourth efferent vessels are closer together, the genus comes under the second subgroup, D b. The circulus cephalicus, also, is less elongated than in Syngnathus. In both genera, however, the front of the circulus is broad and its posterior angle very acute. The dorsal aorta of Hippocampus is median, and the subclavian arteries arise from its sides directly, and not through the intervention of a common root. In Gastrosteus the circulus cephalicus is not more than twice as long as broad. The subclavian arteries arise a short distance behind its posterior angle, and the coeliaco-mesenteric a considerable distance behind. Molva differs from Gadus (fig. 34) mainly in the fact that the subclavian arteries arise from the circulus cephalicus farther from the median line, and consequently more remote from the dorsal aorta. The coeliaco-mesenteric forks close to its origin in both genera, as it also does in Motella. Motella very closely resembles Molva, and it is only the slight separation of the mesial extremities of the last two branchial vessels which causes tbe genera to be placed in separate subgroups. Gadus callarias (G. morrhua), figured by Muller (16. pl. iii. fig. 13) and by Stannius (23. pl. v. fig. 2), does not appear to differ materially from Gadus ceglejinus, |