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Show 710 MR. GERARD W. BUTLER ON THE [Nov. 19, usual, should be found in (some, not all of) the Amphisbaenidae, which are also unique in having the right lung partially or completely suppressed 1. With regard to the lungs of mammals-it has been suggested by some2 that this inequality is due to the unsymmetrical position of the heart. There are, however, certain considerations which induce me to incline to another view 3. Pirstly, the lungs may, as we have seen, differ markedly in size in reptiles in which the heart is symmetrically situated. Secondly, in the few mammals which I have examined the smaller size of the left pleural space seems to depend not so much on the position of the heart as on the want of symmetry in the mediastinal membranes, whose line of attachment to tbe diaphragm is a curve sweeping round the left border of the central tendon. Thus perhaps the first cause of the inequality of the lungs here, as in Snakes, may have been the leftward displacement of the stomach,-which cause, however, may have only come into action when, with the development of the diaphragm, the mediastinum came to be fixed in its oblique left-sided position. According to this view the unsymmetrical position of the heart would be due to the same cause as the inequality of the lungs, and not be itself the cause of this. VIII. CONCLUSIONS. 1. In all the Amphisbaenidae examined the right lung is either absent or smaller than the left. 2. In all the other vertebrates examined the right lung is fully developed, and if one lung is rudimentary or absent, it is the left. Thus 3. The left lung is the smaller in many mammals, and more markedly so in the Gymnophiona and many snake-like Lizards [not Amphisbaenidae] and Snakes, in which last the left is usually reduced to a mere rudiment or absent altogether. 4. In the more theoretical section VII. I incline to the view that in their first beginnings the lungs were in the ancestors of all air-breathing vertebrates potentially paired, having their origin hi paired branchial pouches, and show reason to believe that they were actually paired in the ancestors of at least some forms which show no trace of a second. 5. It would seem that the primary cause of the inequality of the lungs, where it occurs, is that one-sided displacement of the stomach and adjoining portion of the oesophagus which is seen in 1 To avoid needless repetition, other remarks which naturally might follow here are placed only in the next section (Conclusions 5 and 6) 2 G. L. Duvernoy, ' Lecons d'Anat. comp. de G. Cuvier,' 2nd ed. torn. vii. pp. 20, 24, 25 (1840). R. Owen, ' Anatomy of Vertebrates,' vol. iii. p. 577 (speaking of Marsupials) 3 I refer only to tbe leftward displacement of the ventricle. I do not dispute fact that in most mammalian lungs we note that the left bronchus appears the longer, owing apparently to the fact that the one-sided development of the aortic root has entailed tbe suppression of part of the left lung in that region. |