THU PYGOPODES. 151 



the skin. This space also contains air, which it receives through small apertures in the inferior 

 delicate membrane, situated close to the anterior side of each contour-feather. Now, as the small 

 cutaneous muscles between each four contour-feathers are constantly drawn to and fro, and the 

 interspaces of these muscles are also filled up by a delicate membrane which coats the muscles 

 themselves, membranous sacs are formed between them, into which the air can penetrate from 

 the large cavity by the orifices above described. There is not even a trace of fat between th e 

 membranes, which are doubtless formed of delicate layers of cellular tissue, and all the spaces 

 which elsewhere usually contain this matter in abundance here contain only air. I observed 

 precisely the same structure in Pelccanus crispus, which I obtained in spirits from Vienna, and 

 in this case found the spaces, otherwise filled with air, already partly filled with the fluid in 

 which the specimen was immersed. 



5. Phaeton. The pterylosis has no peculiarities, and is very similar to that of Dyspoms, 

 but the narrow spinal space reaches to the caudal pit, and the bands of the dorsal tract do not 

 touch the axillary tracts. On the other hand, the whole posterior dorsal region over the pelvis 

 is uniformly feathered, and on the thighs two rows of stronger contour-feathers make their appear- 

 ance as femoral tracts. In the wing I counted thirty-two remiges and in the tail sixteen rectrices. 

 The oil-gland has a very short, nearly perpendicular mamilla, and three orifices on each side ; 

 of these the anterior is distinctly larger than the posterior ones. Among the many concordances 

 of this genus with the preceding are the pneumaticity of the skin on the breast and the presence 

 of the small air-cells between the contour-feathers. The latter, moreover, are much more elastic 

 and curved than in Tacliypetes, and rather resemble those of the Geese. 



6. Plotus. The Darter in its plumage to a certain extent resembles the Penguins, 

 inasmuch as its body is nearly uniformly clothed with small and rather soft contour-feathers and 

 very delicate down-feathers. The feathers only begin to be more rigid on the sides of the lower 

 part of the neck, but become still more so on the axillary tracts, into which those neck-feathers 

 pass. Besides these only the upper wing-coverts, the primary coverts, the very acute remiges, and 

 the rectrices, are of a stronger formation. The number of rectrices is twelve, that of the remiges 

 is twenty-six. Besides the lateral spaces of the trunk, I found only a narrow inferior space ; all 

 other spaces are wanting. 



5. PYGOPODES. 



Among the Natatores this family is indisputably the most strongly marked pterylographically, 

 both by the form of the tracts and by the texture of its plumage. The latter, wherever feathers 

 stand (and this in many forms is the case over the whole surface of the body except the bill 

 and feet), is denser and more closely packed than in any other bird, and consists, of course, of 

 contour- and down-feathers, except on the spaces where down-feathers alone occur. Nevertheless, 

 both the contour- and down-feathers possess a distinct aftershaft, and this is not wanting 

 even on the nearly scale-like contour-feathers of the Penguins. The object of its presence here is 

 evidently to render the plumage of these birds, which are more constantly in the water than any 

 others, as dense as possible. As regards the forms of the tracts, there are in this family three 

 types, which are very definitely distinguished from each other, and correspond with the groups 

 of the Divers (Colymbus or Podiceps, and Eudytes], the Guillemots (Uria, Alca, Mormon), and the 



