NATURAL SCIENCES OP PHILADELPHIA. 205 



This expression is absolutely diagnostic ; it marks every swimmer, and ex- 

 cludes all other birds. Reference to the bill is necessary to exclude the web- 

 footed Gralke. Nothing can be predicated from the wing-coverts, which are 

 similar in Cursores and Natatores ; nor from the carotids, which are double in 

 liearly all of both sub-classes, the Podkipidw. alone furnishing an exception in 

 Natatores. So with most other single characters useful or indispensable in 

 arranging higher groups. We must chiefly rely upon special comhinalions of 

 common or general characters. 



The statement that all Natatores possess the oil-gland, which has its circlet 

 of feathers, has no known exceptions. The gland usually has two openings ; 

 frequently more among Steganopodes and Procellariidce. Powder-down tracts 

 have not been observed in any. The pterylosis of the sub-class adheres 

 closely to a particular type, in the breadth of the tracts, that are " strong" and 

 definite, and in narrowness of the spaces. The inferior tract, as a rule, has no 

 free outer branch ; the dorsal is not interrupted by a true gap. Laridce fur- 

 nish exceptions to both these statements, in their scolopacine pterylosis ; 

 Diomedea, Colymbus and Podicepa have the dorsal gap ; Uria, Alca, and Fra- 

 tercula have a half-free outer branch reaching to the knee-covert. There is 

 also a trace of this in Anatidie, increased in Procellnriidx. " In all these cases 

 the ventral portion, which is often independent only from the extremity of 

 the sternum, is usually not only very broad but dilated towards the sides in 

 the middle of belly ; a structure which occurs only in water birds, and has 

 already been ver}- distinctly recognized among the Scolopacina'." (Nitzsch.) 

 The contour feathers are placed in regularly parallel, oblique rows, not sepa- 

 rated by a space along the median line of the body. Notwithstanding the 

 denseness of the plumage the ptilosis is not so continuous, even in the pen- 

 guins, as in some Insessorial groups. The spaces are all covered with down- 

 feathers, perhaps double. The contour feathers are so situated that every 

 contiguous four form a rhombus, made quincuncial by a down-feather in the 

 centre. Other smaller down-feathers are frequently intercalated in the rows 

 of contour feathers. The latter are furnished with an extensive series of 

 comparatively large cutaneous muscles, of which each feather may have 

 several. Each contour feather has its accompanying filoplumes, which may 

 be as many as ten, as e. g. in the goose. The modified diversely colored 

 feathers about the head of cormorants (and some auks?) are believed to be 

 filoplumaceous. The feathers are frequenilj^ devoid of after shafts, which 

 appear, in general, to be complementary to down-feathers.* 



The wings of Natatores &vq remarkably diverse in size and shape ; the modi- 

 fications are scarcely or not diagnostic of either orders or families. Thus 

 the wings of certain Procellariidce are exactly those of some Alcidx ; those 

 of some Pygopodes and Lamellirostres are alike. In this sub-class we find all 

 gradations between the two possible extremes ; albatrosses, e. g., are the 

 best flyers of any, and have the most remiges (50) ; penguins have none, af- 

 fording the only instance in the class of abortion of quills ; though other 

 birds share the deprivation of flight. Tachypetes again vies with Diomedea, 

 The Natatorial average of remiges is about 30 ; the number may fall to 25, 

 and frequently ranges up to 40. Notwithstanding this, the primaries are 

 constant, 10 ; the first of these, as a rule, is longest, but there are many ex- 

 ceptions. Macropterous swimmers, as a rule, have flat, narrow, pointed 

 wings ; brachj^pterous, concave, broad rounded ones. Tachypetes and Podiceps 

 best represent the two extremes. Alca impennis furnishes a singular wing ; 

 useless for flight, yet morphologically perfect. The greater wing coverts 

 are always longer than half the secondary remiges. The tail varies as much 

 as the wings, both in shape and character of rectrices. As a general rule, 

 the tail is small and rounded, with many short, rather soft rectrices ; this may 

 be called its typical condition. Abortion of rectrices definitely characterizes 



* This paragraph is compiled from Nitzsch's Pterylography, Ray Soe. Ed. pp. 139-141. 



1869.] 



