1874.] MR. A. H. GARROD ON THE MUSCLES OF BIRDS. Ill 



1. On certain Muscles of Birds and their Value in Classifi- 

 cation. Part II. By A. H. Garrod, B.A., Fellow of 

 St. John's College, Cambridge, Prosector to the Society. 



[Received January 6, 1873]. 



(Plate XVII.) 



The facts contained in the former part of this communication 

 (P. Z. S. 1873, p. 626) being in an expanded form, it is not easy to 

 appreciate their full significance at a glance, nor without considerable 

 difficulty. To obviate this inconvenience I have constructed the 

 following table, which is so arranged that by a very simple method, 

 it is possible to tell without further reference which of the five 

 muscles — the ambiens, the femoro-caudal, the accessory femoro- 

 caudal, the semitendinosus, and the accessory semitendinosus — are 

 present or absent. To obtain this result the names of the muscles 

 themselves have been omitted, and single letters of the alphabet 

 used in their stead. 



The femoro-caudal is represented by A 



The accessory femoro-caudal B 



The semitendinosus X 



The accessory semitendinosus Y 



When these four muscles are present in a bird the formula 

 A B. X Y expresses the fact ; when any one is absent, that such is 

 the case is indicated by the omission of the letter representing it. 

 Thus the formula A. X Y indicates that the accessory femoro-caudal 

 muscle only is absent ; A B. X that the accessory semitendinosus is 

 missing ; A. X that the femoro-caudal and semitendinosus only are 

 to be found ; and A that the femoro-caudal alone is present. These 

 formulae may be termed myological formulae. No bird is known in 

 which all these four muscles are deficient. 



In the accompanying diagram (Plate XVII.) all those birds which 

 have the same myological formula are included together in one circle ; 

 and the circles are so placed in relation to one another that, if they 

 were drawn on the surface of a sphere, there would be only a single 

 operation of addition or subtraction necessary to move from any one 

 to any of its immediate neighbours. 



Of the four letters A, B, X, and Y the following are the sixteen 

 possible combinations. 



AB.XY AB B.XT XT 



AB.X A.X B.X X 



AB. Y A. Y B. Y Y 



A. XT A B 



Those printed in italics are those which, if considered as myo- 

 logical formulae on the nomenclature above given, have been 

 found amongst birds ; in other words, there are eight different 



