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Show 52 MR. F. E. BEDDARD ON THE [Jan. U , reason of the marked separation of the two ventricles (not so marked, however, as in the Dugong), and by the conspicuousness of the coronary arteries which ramify over its surface; the organ is also, as has been remarked, small for the size of its possessor. I found that in M. Inunguis the ductus arteriosus was perfectly permeable and was a tube of some size; in the heart of a young M. latirostrls a little more than half its size, the ductus was tubelike in form and had an orifice into the pulmonary artery, but was apparently not permeable throughout. In the M. latirostrls with a heart twice the size, the ductus was absent or rather represented by a flat non-tubular ligament. This seems to indicate a specific difference between the two forms in the relative age at which the ductus arteriosus degenerates. But it is, on the other hand, quite likely that there are variations in respect of this. The right ventricle in all three specimens was less sculptured on its inner surface than the left. It has in all three a moderator band, which is most pronounced in M. Inunguis. This moderator band in that species is continued forwards after it has become adherent to the ventricular wall to the base of one of the semilunar valves of the pulmonary artery. In the young M. latirostrls this was so to a less extent; while in the larger M. latirostrls the moderator band was free from the heart-wall for a less extent and did not run forwards to the base of the valve at all. I observed no differences in the mode of origin of the principal arteries of the aorta in the three specimens. The brain of Manatus latirostrls has been described and figured by Murie1, Chapman2, Garrod3, and by Murie4 again. Of these the last-mentioned paper appears to m e to contain the best figures. I have in m y possession a brain of that species which differs slightly from previously described brains ; the drawing (fig. 5, p. 53) will also enable m e to compare more satisfactorily the two species in respect of their brain structure. This organ in fact shows certain slight but characteristic differences in the two Manatees. In the first place there is the shape when the brain is viewed from above: in M. inunguis the cerebral hemispheres are markedly longer in proportion to their breadth, the two dimensions being, as nearly as I could measure, 65 m m . and 30 mm.; on the other hand, in m y specimen of M. latirostrls the same measurements were 65 m m . and 37 m m. about the same as those of the brain figured by Garrod. The outline of each hemisphere is more distinctly C-shaped in M. Inunguis, the C's being of course back to back. The only fissures of the hemispheres in M. Inunguis are the Sylvian, both of which are Y-shaped above. Seen laterally each fissure runs forward near to its termination; but the forwardly directed part of the fissure is, in my opinion, the rhinal fissure, which does not, at least so 1 " On the Form and Structure of the Manatee," Tr. Z. S. viii. p. 627 2 "Observations on the Structure of the Manatee," Proc Acad W»t s™ Philad. 1875, fig. 452. ' ' l' atl' 3 "Notes on the Manatee, &c." Tr. Z. S. x. p. 137. 4 " Further Observations on the Manatee," ibid. xi. p. 19. |