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Show 1873.] MR. GARROD ON THE NASAL BONES OF BIRDS. 33 It is not easy to understand the causes of the geographical distribution of many marine animals. Temperature and depth of water have no doubt much to do with it in many cases, as, for instance, with the various kinds of corals ; and such causes may have their influence on the range of this sponge ; but it is interesting to note that of the two known localities for it, one of them (Torres Straits) is in lat. 10° S., and the other (the Ceylon pearl-banks) is in lat. 9° N . The temperature of the Ceylon seas varies but little from 82° Fahr.; and that is, I believe, about the warmth of tropical waters in general, unless influenced by some polar current. The apparent absence of this sponge from the intermediate equatorial sea is therefore due probably to the little use that has yet been made of the dredge in the waters between India and Australia, rather than to any difference in the physical conditions of life there ; and if the Deep-Sea Dredging-Expedition does not meet with it in that as yet little-explored region, the localization of the genus Xenospongia at short and almost equal distances north and south of the equator will be rather remarkable. * This sponge is not mentioned by Dr. Bowerbank in his report on m y collection of Ceylonese species, as the specimen was sent to the British Museum, and did not come into his hands for examination. 4. O n the Value in Classification of a Peculiarity in the Anterior Margin of the Nasal Bones of certain Birds. B y A . H . G A R R O D , B.A., F.Z.S., Prosector to the Society. [Eeceived December 3, 1872.] Since commencing the study of the anatomy of birds, it has always appeared to m e that two distinct types of nasal bones can be distinguished among them without difficulty-and that if those which present the abnormal characters are considered separately, they present other features in common which justify their being placed in the same class, and their entire separation from those which present the less-modified arrangement. In most birds the anterior margin of the nasal bone is concave, with the two cornua directed forwards-one along the outer edge of the nasal splint of the prsemaxilla, to form the inner margin of the osseous external nares, whilst the other, which is free, descends as part of the external boundary of the same aperture in connexion with the ascending process of the maxilla, which it joins. These two processes become continuous behind with the body of the bone, and with one another, there being no interruption of any kind between them. Such a condition is found in its simplest form in Otis and the Gallinse proper; and birds possessing the bone so constructed may be termed holorhinal: in them a transverse straight line, drawn on the skull from the most backward point of the external narial aperture of one side to that of the other, always P R O C . Z O O L . S o c - 1 8 7 3 , No. III. 3 |