14 THE VOYAGE OF H.M.S. CHALLENGER. 



inspection, there is a slight elevation visible on the area usually occupied by the hallux, 

 and on dissecting away the skin a single 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 exulans it has 

 a length of 0*2 inch (5 mm.) ; in the other two species, particularly in Thalassiarche, 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 how- 

 ever not extending to the hallux. The claws are well developed ; in the Procellariidaj 

 they are always more or less curved, compressed, and sharp-pointed (vide PI. I. fig. 5, b, 

 Procellaria pelagica), whilst in the Oceanitidse they become lamellar, depressed and 

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

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

 tarsal scutellation and the form of the claws, whether it is one of the Oceanitida3 or of 

 the Procellariidse. 



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. 



As regards the number of rectrices in the Tubinares, twelve is the ordinary number 

 in both families, and this is never reduced, 1 and only in a few instances exceeded. 

 Fulmarus and Daption have fourteen tail feathers, as already pointed out by Nitzsch, 

 and the same is the case in Thcdassceca ; Ossifraga has as many as sixteen. Aeipetes 

 antarcticus (in both the specimens examined by me) has, on the other hand, unlike 

 Thalassceca, the normal number of twelve. 



The number of primary remiges is always ten, but that of the secondaries varies. 

 The number of these in the Oceanitidas is always ten : in the Procellariidae it is never, even 

 in the smallest forms (Pelecano'ides, Cymochorea, Halocyptena, Procellaria), less than 

 thirteen. Bidweria has twenty, which is about the average number throughout the 

 group, increasing however in the larger forms to twenty-nine (Ossifraga, Thalassiarche), 

 thirty (Diomedea brachyura), and even, in the largest of all, Diomedea exulans, to 

 thirty-seven. 2 The pollex never has the claw so often present in birds on that digit. 



As regards the distribution of the tracts of contour feathers, I may epiote Nitzsch 's 

 general description (Pterylography, Pay. Soc. Ed., pp. 143, 144): — "In this family the 

 tract-formation of Lestris is elevated into the type of a group, undergoing scarcely any 

 change in the form of the inferior tract, but showing some little modification in the dorsal 



1 Nitzsch (Pterylogr. Ray Soc. E<1, p. 141) thought that the smaller species of Petrels had but ten tail feathers, 

 but such is not in reality the case. 



2 Nitzsch even describes the species as having as many as forty secondaries. The total alar expanse of the specimen 

 I counted this number in was 9 feet 7| inch. 



