THE PHYLOGENETIC SIGNIFICANCE 

 OF THE PREHALLUX AND PRE- 

 POLLEX: A THEORY. 



Bj' C. D. Gillies, M.Sc, and P. W. Hopkins, M.C. 



(Plate I.) 



{Read before the Royal Society of Qiieensland, 27th June, 1921.) 



One of the most conspicuous discontinuities in the accepted 

 evolutionary scheme of the Tetrapodal Vertebrata lies between 

 the Fish on one hand and the Amphibia on the other ; this 

 hiatus is accentuated by the almost dramatic appearance of 

 the pentadactyl limb, or cheiropterygium, in its typical form 

 in the earliest laioAvn Tetrapoda, i.e. the Stegocephaha, and, 

 to add to the obscurity, embr3'ology has thrown but little 

 illumination upon the derivation of the appendage from the 

 Fish hmb or ichthyopterygium. 



Those fish which are regarded as being nearest the Tetra- 

 poda, such as the Dipnoi and Crossopterygii, possess pelvic and 

 pectoral appendages typically piscine, whereas the most primi- 

 tive Amphibia are provided with limbs constructed upon the 

 pentadactyl plan, in common with the higher Tetrapoda. 

 Omitting the girdle, the cheiropterygium in its typical form is 

 usually resolved into five parts, viz. : — 



(a) A proximal single long bone (e.g. femur) ; 



(6) A set of two long bones (e.g. tibia and fibula), which 

 articulate with 



(c) A series of small bones (e.g. tarsalia), followed by - 



(c?) A set of five small elongated bones (e.g. metatarsals), 

 supporting 



(e) Phalanges of the digits. 



For a considerable period all the metacarpals and meta- 

 tarsals were regarded as being serially homologous in each 

 instance, but it has been subsequently shown that the so-called 

 first metacarpal and metatarsal are both homologous \Adth 

 phalanges. This being so, the question suggests itself, What 

 has happened to the true first metacarpal and metatarsal ? 

 The fate of these structures has engaged our attention for some 

 time and, as a result, in this paper is presented a theory which 

 we hope will satisfactorily explain the disappearance of these 

 bones. 



So far as we can ascertain from the available literature we 

 believe that we have not been anticipated in our views. 



