1873.] MR. GARROD ON THK 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 

 my collection of Ceylonese species, as the specimen was sent to the 

 British Museum, and did not come into his hands for examination. 



4. On the Value in Classification of a Peculiarity in the 

 Anterior Margin of the Nasal Bones of certain Birds. 

 By A. H. Garrod, B.A., F.Z.S., Prosector to the Society. 



[Keceived December 3, 1872.] 



Since commencing the study of the anatomy of birds, it has 

 always appeared to me 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 Gallinae 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 



Proc. Zool. Soc— 1873, No. III. 3 



