48 PEOF. W. K. PAEKEE OX THE 



birds and the long-billed Shore-birds (Limicolse), the bill rapidly elongates and takes 

 on special curves, up or down, after hatching ; these are comparatively recent 

 specializations. But in some other kinds, e. g. the Guillemot [Uria troile), the 

 bill tends to become as long as it is in the toothed birds of the Cretaceous Epoch. 

 By the middle of incubation, the parosteal tracts first dominate, and then largely take 

 the place of the endoskeletal elements, clasping them and stopping their growth. For 

 a time, the cartilaginous rods struggle to grow to the ancestral length, but in this effort 

 they become bent and deformed ; this is all put right by the time of hatching ; they 

 cease to grow, overmastered by the enveloping splint-bones. But this twisting, and 

 then arrest, of the cartilaginous rods appears also where there are no splint-bones, but 

 merely the nerves, muscles, and ligaments to which these parts are related. This 

 curious quasi-deformity is seen, not only in Uria troile, but I shall soon show and 

 describe it here, in Opisthocomus ; it appears in the pubic rods and in the columella 

 auris. Of all existing birds, the African Ostrich comes nearest the Iguanodon and its 

 kindred in the size of the rotated pubes ^. 



There has evidently been, first, a secular rotation and elongation of the pubis and 

 ischium, and then a re-shortening of these parts in the higher kinds of birds. A 

 similar phenomenon is seen in the length of the sacrum, and correlatively of the extent 

 of the ilium, behind and before. The African Ostrich and the Swan have each twice, 

 or nearly twice, as many sacral vertebrae as the lesser birds of the higher kinds, both 

 Passerine and Cuculine ; and these lesser forms constitute about half the known 

 existing birds. Even in the Rook [C'orvus frugilegus), one of the chief of the Goraco- 

 morphae, and in that Order a large bird, there are only eleven sacrals in the adult ; 

 whilst there are twenty-one, and sometimes twenty-two, in the common Swan and in 

 the African Ostrich. 



The young Rook, after it is fledged, has a longer sacrum than the adult ; there is, 

 for a time, a twelfth vertebra in that part of the spine ; the ancestral Crows had, 

 probably, a longer sacrum than the existing forms. At present, with a large number 

 of unclassifiable facts in hand, it is safer for us to confess our ignorance as to how Birds 

 and Mammals arose, than to invent crude hypotheses that will only be mocked at by 

 those who enter into our labours in the time to come. Of one thing I feel certain, 

 even now, and that is that no feathered or hairy form ever arose from a true gill-less 

 Amniotic Reptile. A very large proportion of the Reptiles that have arisen during 

 the geological ages have died out; these all came short, as the existing reptiles now 

 come short, of the high excellences of the hot-blooded types, feathered or hairy. 



II. The Early Stages of Opisthocomus cristatus. 

 There were three different stages in the four specimens sent to me ; these, being 

 measured, gave the following lengths from the end of the beak to the end of the tail: — 

 ' bee DoUo, on Iijuanodon hernissarUnsis, Blgr. Bull. 5Ius. Eoy. Hist. 'Sat. Belg. t. ii. pi. t. 



