MOEPHOLOGT OF OPISTHOCOMUS CEISTATUS. 77 



both ends ; the same thing is seen in Corythaix ; in Opisthocomus there is a clear space 

 between these parts, right and left ; this is also seen in the Turnicidee. Even this part 

 of the skeleton, therefore, like the rest, suggests a low ornithological position for this 

 bird, and justifies the use of the term archaic as applied to it. 



VIII. The Hind lAmh of Opisthocomus cristatus. 



The leg of Oj)istkocomus in the 1st stage is strong and rather stout (Pi. VII.) ; its 

 long elements have a considerable ectosteal sheath ossifying the solid cartilage. The 

 parts of this limb difi'er little from those of an embryo of the Common Fowl at this 

 stawe ; we shall find a few notable difierences, showing that we are, here, at the parting 

 of the ways. The relative length of the main divisions of the limb, at this stage, is 

 very similar to what is seen in a large number of those Carinatae that have moderately 

 long legs, whether they are Perchers, Walkers, or Runners. Measured from the chief 

 points of flexure, these parts have now their length as below : — 



Pemur. Tarso-tibia. Tarso-metatarsus. 3rd toe. 



11 millim. 13 millim. 8 miUim. 14 millim. 



The ends of the large cartilages are still unossified, and they will not be finished by 

 epiphyses, except in the case of the cnemial process of the tibia : in a few CucuUne 

 birds the top of the fibula has a small epiphysis, but that is rare in the Carinatae ; in 

 the fore limb, in some rare cases, sometimes the radius and sometimes the ulna has an 

 epiphysis. The cnemial process is not large ; the fibula (fb.) has shrunk to half the 

 length of the tibia (t.); the tibia is quite distinct from the proximal tarsals (tb., 

 fbe.). All the true tarsal elements are free from bony deposit at present (Pis. VII. & 

 X. fio-. 10). There is some contention as yet as to the true nature of these segments ; 

 but, after much labour and thought, my own mind is made up as to their morpho- 

 logical meaning. When I fii'st asked whether or no the lower part of the tibia was an 

 epiphysis or " the homologue of the Mammalian astragalus " (" On Balmniceps rex" 

 Tr. Z. S. vol. iv. p. 343), I was still in the dark as to the nature of the " sesamoid os 

 calcis." Since then, several anatomists, notably Gegenbaur and Huxley, have worked 

 at this part of the bird's skeleton. 



One of the most valuable pieces of work on this subject is Dr. Morse's paper " On 

 the Intermedium in Birds" (Ann. Mem. Boston Nat. Hist. 1880), though his views, 

 which are mine also, have been controverted by Dr. G. Baur\ 



In a paper (ready for publication), my son, Prof. T. J. Parker, also holds the same 

 views as Dr. Baur; his observations have been made on the development of the 

 Aptenjx. I am perfectly convinced of the truth of Dr. Morse's views. Nevertheless 

 I believe that there are only two morphological elements in the proximal tarsal series — 

 an astragalus, or tibiale, and a calcaneum, or fibulare. The so-called sesamoid, or 



' See his paper, " Der Tarsus der Vogel imd Dinosauiier," Morph. Jahrb. Ed. viii. pp. 417-456, Taf. 19, 20. 



m2 



