40 PTERYLOGRAPHY. 



apart, and being either down-feathers or, at least in part, stiffer contour-feathers. I have found 

 such contour-feathers, mixed with downy plumes, largest and most abundant upon the oil-gland 

 of Diomedea exulans. 



In the general form of the gland I find the following chief differences : 



Nearly triangular in Vultur leucocephalus, Falco milvus,Lanius minor, and most of the Passerine. 



Truly cordate, and consequently not longer than broad, in most of the Diurnal Rapacious 

 Birds, in Slrix nyctea and S. Jlammea, Cypselus opus, Picus viridis, Junx torquilla, Musophaga 

 paulina, Alcedo ispida, (Edicncmus crepitans, and many others. 



Elongato-cordate in some Owls, such as Striz aluco and S. otus ; also in the Common Pigeon, 

 and in Scolopax rusticula. 



Obtusely cordate, that is to say, bilobed anteriorly and truncate behind, in Ciconia, Larus, 

 Sterna, and Halieus. 



Deeply bilobed, the two halves being widely separated, and only united at the extremity, in 

 Cuculus, Coccyffius, Picus martius, Psittacus aracunya and ararauna, and in the Ducks and 

 some of the Mergansers. 



Transversely reniform in Sturnus, and very similar to this in many other Passerines. 



Very broad, transversely elliptical, in Tetrao tetrix. 



Cordato-elliptical, longer than broad, in Perdix coturnix, Dysporus bassanus, and Pelecanus 

 crispus. 



There are, however, some forms which occupy a middle place between those just described, 

 and even variations among different individuals of the same species, apparently dependent on 

 differences of age, sex, or individual conformation. 



It is usually thick and convex in the Aquatic Birds, although even in many of these it is flat ; 

 it is flattest in Dysporus. 



In Slrix Jlammea and Caprimulyus europaus I find it nearly erect, so that only the hinder 

 part lies upon the tail. 



In many instances the efferent duct is not distinctly separated from the gland, but passes 

 gradually into it, by becoming thickened anteriorly ; in other cases it is very thin from its com- 

 mencement, and readily distinguishable from the body of the gland. I find this latter form in all 

 the Passerines, and in Upupa, in which the efferent duct forms a hollow, nearly pyriform body, in 

 some of the Gallinacea and Ducks, and in Scolopax rusticula ; the former structure is the usual 

 one in all "other birds. Thus, the duct is usually produced by the gradual posterior elongation of 

 the two halves of the gland, but it also issues from the posterior surface of the gland, and then 

 the body of the gland at the base of the duct is more or less elevated, as I have seen it in the 

 Passerines, and in Tetrao, Perdix, and Colymbus. 



In Dysporus and Pelecanus this process is entirely wanting, and in these I find in each of 

 the flat halves of the gland, which are truncated behind, a peculiar orifice, situated before the 

 apical margin ; this is simple in Dysporus, but consists of several small apertures in Pelecanus. 

 Moreover, even when the above-mentioned process is not deficient the external orifice is double, 

 each half of the gland having its own aperture. The two apertures may generally be dis- 

 tinguished with ease ; but in the Passerine, the Owls, and perhaps generally in all birds of which 

 the outlet has no circlet of feathers, they are so small and placed so close together that their 

 recognition is difficult. In Upupa epops, in which, as I have already mentioned, the efferent 



