THE VOYAGE OF THE 4 CHALLENGER.' 377 



the Procellariidae of similar size, but is always present * and very straight 

 and spur-like. In most of the Procellariidae it is larger and more curved : 

 it is best developed proportionately, perhaps, in Pagodroma. 



In the Albatrosses the hind toe is so minute that these birds are 

 usually described as being three-toed, but this is not really quite correct. 

 In Phcebetria the hallux externally only just appears, being represented 

 merely by a slight pimple-like elevation, with a very minute claw. 

 On dissecting away the skin, the pimple is seen to be connected with 

 two minute bony nodules, the basal one, which represents the metatarsal, 

 more globular, the apical one more pointed and covered by the minute 

 claw. They are only connected by connective and fibrous tissue to the 

 tarso-metatarsus t, and are separated from, each other by a considerable 

 interspace, the whole having a total extent of only 3 mm. (vide fig. 2, a). 



In Thalassiarche (culminata) and Diomedea (brachyura and exulans^thia 

 hallux is still more rudimentary, and there is not a trace of a nail outside. 

 Still, on careful inspection, there is a slight elevation visible on the area Zool. Chall 

 usually occupied by the hallux, and on dissecting away the skin a single t^^p 14 

 minute ossicle, of triangular shape, is to be found attached to the tarso- 

 metatarsus and surrounding structures by fibrous tissue. This single 

 bone probably represents the metatarsal element. In Diomedea eoculans 

 it has a length of 0'2 inch (5 mm.) ; in the other two species, particularly 

 in ThalassiarcTie, it is much smaller, not exceeding here 2 mm. in length 

 (vide figs. b-d). I have been unable to find it at all in Pelecano'ides, even 

 in quite young birds. 



The anterior three toes are well developed, and are completely webbed, 

 the web, however, not extending to the hallux. The claws are well 

 developed; in the Procellariidaa they are always more or less curved, 

 compressed, and sharp-pointed (vide PL XIII. fig. 2, b, Procellaria pelagica), 

 whilst in the OceanitidsB they become lamellar, depressed, and flattened, 

 a shape that attains its maximum in the genus Fregetta (vide figs. 1-4, 6). 

 Hence an inspection of the legs alone of a Petrel will show, by the 

 character of the tar sal scutellation and the form of the claws, whether it 

 is one of the Oceanitidae or of the Procellariidae. 



Pterylosis. The pterylosis of this group seems to be, on the whole, 

 very uniform throughout, both in the form of the tracts and the structure 

 of the feathers. 



The number of rectrices and remiges is not absolutely constant 

 however, and there are also some slight differences in the form of the 

 dorsal and lumbar tracts. 



* Mr. Dresser erroneously describes it as wanting in Oceanites (Birds of Europe, 

 vol. viii. p. 503). 



t The existence of the rudimentary hallux in Pkoebetria fuliginosa was first, I believe, 

 pointed out by Dr. Kidder in his account of the birds of Kerguelen's Land, Bull. IT. S. - 

 Nat. Mus. vol. i. p. 22. 



