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Show 20 DR. J. F. GEMMILL OX [May 12, press them widely apart. The growth in bulk of this mass which causes this pressure can only have occurred at a relatively late stage. Almost perfect coalescence of the medullar has thus been allowed through secondary fusion, while the upper parts of the spinal cords have been kept markedly composite. Further, there is reason to believe that nearly all the composite mesenchymal structures occurring at the transitional region have been produced through secondary fusion. The monstrosities grouped under Class III. (p. 16) do not exhibit symmetrical lateral union, owing to their marked ventral convergence, and, accordingly, what has been said above regarding the behaviour of different oigans at the transitional region does not apply to this Class. In all cases except those of pure ventral union (for an example of which see p. 18), the change from the double to the single condition is effected at the expense of the inner or adjacent elements, while the outer elements become the right and left sides of the single region of the body. This rule has long been recognised as holding good in similar double monstrosities among the higher vertebrates. For the sake of easy reference, it may be useful to tabulate here the principal composite structure, produced by fusion of adjacent elements, and mentioned in the descriptive part of this paper :- Trabecular cranii, Parachordal cartilages, Vertebral do., Palato-quadrate do., Supraorbital do., Periotic do., Hyomandibular do., Meckelian do., Hyoid do., Pectoral fin do., Jugular veins, Cardinal veins, Pseudobranch arteries, Pronephric glomerulus, Wolffian ducts, Muscles, Eyes, Internal ear, Optic lobes, Medulla oblongata, Spinal cord, Cranial nerves, page 7, Plate I., fig. 3, d. 11 11 n 11 n n 11' 11 11 n n 7,11, 8, 8,11, 8,11, H, H, 8,12, figs. 3, 5, c. fig. 8, Na', Ha'. figs. 3, 5,/'. figs. 4, 6, *'. fig. 5, k'. fig. 5, i'. ng. 3, g', 9,10, Meek'. 12, Plate II., fig. 10, LH',BH'. 15, „ fig. 11, CL. 15, Plate III., fig. 19, DC. 15, „ fig. 20, VC. 9, Plate II., fig. 12, A.Chor', fig. 13, A a. „ 10,16. Plate IV., figs. 29, 31, 32. „ 16, „ figs. 31, 32. 9 )) 11, Plate IV., fig. 27. „ 8, Plate II., figs. 15, 17. ,, 12, „ fig. 16. „ 9, 12. „ 13. As a rule, where adjacent muscles persist, there are also corresponding motor nerves. But the small muscles described as |