Pterylograpliy of the Hoatzin. 359 



losis and in the distribution of the feathers of the spinal tract 

 in particular. Thus, in Turacus and Centropus, for instance^ 

 the pt. spinalis has run down and fused with the pt. femoralis, 

 whilst in Opisthocomus that portion of the apt. trunci late- 

 ralis dividing these tracts is sparsely covered with both 

 down-feathers and semiplumes. Now it seems to me that 

 these semiplumes are to be regarded as degenerated contour- 

 feathers^, as was suggested long ago by Garrod (see p. 366), 

 the evident remains of a once continuously feathered area. 

 Again, there is a striking similarity in the pterylosis of the 

 wing of Turacus and Opisthocomus. We may count as 

 Rallinef, or perhaps Gruiform, the structure of the ^"^neos- 

 soptiles"' (p. 355), since this agrees precisely with that of 

 Crex, for instance, but is entirely different from the Galliform 

 neossoptile, with its well-developed rhachis and aftershaft. 

 The presence of down both on the pterylse and apteria is 

 another Gruiform character. The position of the external 

 nares might, perhaps, be taken into consideration here — if 

 it have any weight at all. But, in that we can divide the 

 nestling-plumage into neossoptiles and plumulae, OpistJw- 

 comus appears to be unique. This is a matter, however, for 

 further investigation, though up to the present plumuhe 

 have never been described as forming part of the plumage of 

 the nestling, Avhich has always been supposed to consist of 

 neossoptiles (" nestling-down ") only. 



The nestling makes up for any lack of novelty that may 

 be deemed to be wanting in the adult. 



The young of Opisthocomus are nidifugous, and, as we have 

 already seen, sparsely clothed in a covering of fine down, 

 which, in its thickness, would seem to be somewhat inter- 

 mediate between that of the Pigeon on the one hand and 

 the Fowl or Rail on the other. This downy covering is 

 formed in the same place as the future definitive feathers. 

 Dr. Gadow has recently aptly named these " nestling down- " 



* The conditions which obtain at a certain stao^e of development in the 

 embryo tend to support this view (see p. 363). 



t I speak guardedly here, as I have not yet had an opportunity of 

 examining a .series either of nestlings ur adults of this group. 



