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Show 10 DR. J. F. GEMMILL ON [May 12, As is well known through the work mainly of Johannes Miiller (' Vergleichende Anatomie der Myxinoiden'), the pseudobranch and the choroidal gland are so related to one another in osseous fishes that a pseudobranch is never present in species which have no choroidal gland, while in rare instances only is a choroidal gland present where there is no pseudobranch. It is therefore of considerable interest to note that the choroidal glands of the inner or adjacent eyes, in the type of monstrosity under consideration, derive their blood-supply directly from the first aortic root. There is a certain amount of variation in the exact mode of origin of the choroidal and carotid arteries. For instance, cases occurred in which these vessels all arose from the first aortic root on one side only, instead of both roots participating equally as in the specimen figured (PL II. fig. 12). As regards the alimentary canal, there are two mouths, but the pharynx and the rest of the canal are single, the only evidence of duplicity being the presence of two air-bladder diverticula. The mesonephros, ureters, bladder, and urinary pore are normal, but the pronephric glomerulus is composite, or may be double. An example of the composite condition is figured in PL IV. fig. 29. The glomerulus is larger than in the normal case, and contains two vascular tufts between which is a median compartment that obviously corresponds to the fused adjacent halves of a pair of glomeruli, but has no Wolffian ducts in connection with it. In the specimen from which figure 30 (PL IV.) was taken the two glomeruli are separate, but one of them has only a rudimentary Wolffian duct arising from its inner side, while the other has none at all. Fig. 29 (PL IV.) should be compared with figs. 31 and 32 on the same Plate, which show a still greater degree of duplicity in the pronephros. Of composite muscles, the most important are contained in the median mass mentioned on page 9 as underlying the twin notochords, and innervated by the small extra motor roots of the composite spinal cord. This muscular mass is segmented serially by septa which correspond exactly with the septa of the outer (normal) lateral muscles. In the head-region some small and intricately arranged muscles are found connected with the cartilages which represent the reduced adjacent Meckelian and palato-quadrate bars. These muscles are obviously rudiments of adjacent mandibular and temporal muscles. It has been noted previously that there are no external rectus muscles in connection with the adjacent eyes, and that the superior obliques are either rudimentary or entirely absent. A typical specimen of the second subgroup of Class I. will next be described, i. e. a specimen which shows the twin brains uniting at the medulla oblongata. The structure of the cranial skeleton in this type agrees with that of the monstrosity last described, except that the place of union of the skeletal elements is carried further back. This gives |