MOEPHOLOGT OF THE GALLINACE^. 239 



developmeut of the cncmial process ; iu this young bird the epiphysis has not yet 

 appeared on the head of the tibia (i.). At its lower end the three remarkable additions 

 to this bone are in an instructive stage of development, for the cartilaginous mass now 

 shows three distinct ossifications — the tibiale, fibulare, and intermedium {tb., fbe., L). 

 The latter, as in the Common Eowl, is very short ; it is obliquely placed and notched on 

 its broad upper part ; it is an ectosteal tract. The other two are endosteal tracts, formed 

 later than the intermedium ; the tibiale is twice the size of the fibulare ; they are botli 

 largely coated with cartilage, the superficial part of which will remain on the bilobate 

 condyle. It is evident that the earlier-formed intermedium cannot be a mere ossification 

 in an " outgrowth " of this condyloid mass ; it is, however, instructively like the part 

 which in the Dinosaurs has manifestly become one with the tibiale or astragalus (see 

 DoUo on Iguanodou, Bull. Mus. Roy. Hist. Nat. Belg. tom. ii. plate 4. fig. 2, e, g.). The 

 fibula {fb.) is more than two-thirds the height of the tibia. Unlike that of the Common 

 Fowl, the centrale or " scaphoid " (fig. 8, c) is small ; it is not yet ossified. The distal 

 tarsal mass {d.t.) has a limpet-shaped osseous centre in it; this is the tarsal which 

 belongs to the largest or 3rd metatarsal which carries the largest toe ; this bony part is 

 growing into the cartilage right and left, just as the cartilage itself grew at first ; the 

 bony deposit is a perfect imitation of the primary cartilaginous growth. This does not 

 always take place, and yet the two tissue-stages are intimately related. Of course, in 

 many cases the cartilaginous matrix fails to undergo the normal segmentation, which 

 may, nevertheless, assert itself in the formation of bony centres. 



In this young bird we see what is still more remarkably seen in the young of long- 

 legged Wading-birds, namely, the clumsiness, so to speak, of the compound shank. Tiie 

 2nd and 4th metatarsals (fig. 8, mt^-, mt}) have squeezed the 3rd {mt:'') backwards, where 

 it supports the special outgrowth of the distal tarsal mass ; below, this rod comes to the 

 front, and is the largest of the three developed rods. In this bird it carries a very large 

 middle toe {dy:') ; all these toes are quite normal, both from a reptilian as well as an 

 avian standpoint. The rudiment of the 5th metatarsal is not now evident ; that of the 

 1st (figs. 9-11, mt}) is not always present in this species. I have been able to study 

 this aborted element in three individuals from the same batch of nestlings. In one 

 (fig. 9 a) I failed to find any remnant ; in another (fig. 9) it was a very small oblong 

 ossicle ; in a third (figs. 10, 11) it was larger and kidney-shaped. 



This abortive development of the hallux or 1st digit in the Hemipods gives them a 

 Plover-like appearance, but it is quite as much a Struthious as a Charadriine character. 

 As most of the Ratitse have lost this digit, its abortion in this case is quite similar to 

 what we find in many Water-birds, which, when three-toed, are seen to show some 

 remnant of the hallux. 



XV. — Summary. 



Amongst the higher Vertebrata, the Bird Class is by far the most potent in genera and 

 species, about twelve thousand species being found in the existing avifauna. 



If, instead of this abundance of living forms, this were an extinct Class and we knew 



36* 



