THE VOYAGE OF T11E ' CHALLENGER.' 429 



aftershaft in these birds, disposes of two of the characters which have 

 hitherto been available for their separation from the other Tubinares, as 

 do the gradations of form that exist in the amount of separation of the 

 two parts of the dorsal tract of another. As peculiarities of the Dio- 

 uiedeinae may be included : 



The lateral position of the nostrils*. The presence of a distinct Zool. Chall. 

 gluteus quintus muscle. The formation of the biceps humeri muscle, pt^^p 57" 

 which gives off a patagial slip from its coracoidal head. The characteristic 

 sternum. The absence of haemapophyses on the dorsal vertebrae. The 

 pneumatic os humeri. The generally pneumatic condition of the skeleton. 

 The proportion of the mauus to the humerus and ulna. 



The tongue and palate are also more or less peculiar, and in all 

 the genera there are uncinate bones, no basipterygoid facets, and two 

 large distinct accessory wing-ossicles ; the right liver-lobe is also distinctly 

 the larger of the two. 



There are apparently three good genera of Albatrosses, which may be 

 distinguished, independently of external characters, as follows : 



Diomedea. Tongue very short ; uncinate bones more or less styliform. 

 (Diomedea exulans and bracJiyura.) 



Thalassiarclie. Tongue intermediate ; uncinate bones styliform. 

 ( Thalassiarche culminata.) 



Phcebetria. Tongue much longer; uncinate bones flattened; hallux 

 better developed than in the other genera, and with an external claw. 

 (Phcebetria fuliginosa.} 



Neglecting for the present the peculiar diving Pelecanoides, the re- 

 mainder of the Procellariidae forms a natural group distinguished by the 

 following characters from the Albatrosses (Diomedeinae) : 



The more or less dorsal position of the nostrils, the form of which, 

 however, varies, as has already been described, though they are never 

 lateral. The absence of a glutens quintus. The peculiar form of the 

 biceps brachii muscle, which is in two separate parts, the humeral head 

 forming a patagial slip. The presence of haemapophyses on the dorsal 

 vertebrae, the centra of which are marked by more or less developed pneu- 

 matic depressions. The non-pneumatic humerus. The different ptery- 

 losis, and the nearly equal size of the lobes of the liver. The greater 

 size of the hallux, which always has a distinct nail externally. (Quite 

 absent in Pelecano'ides.} 



* This feature, in which the Albatrosses are apparently more primitive than are 

 either the Oceanitidse or the other Procellariidae, can hardly, if my views about the 

 relationships of these groups to each other be correct, be considered to have been a 

 character of the common Petrel-ancestor. It may be more probably explained as due 

 to arrested development during embryonic life, as a study of the development of the 

 nostrils of other Petrels would probably show that these are actually, at some time, 

 lateral, and subsequently coalesce. 



