13 Mr. W. A. Forbes on the late Professor Garrod's 



sorius, wliicli arises from the external surface of the pelvis 

 behind the femur^ and joins the main muscle, to be inserted 

 with it into the femur, but nearer the head of that bone. 



As already mentioned, some birds possess all these six 

 muscles, but no bird is known which does not possess at least 

 one. It is a convenient plan to designate the four last-named 

 muscles by the letters X, Y, A, B. Thus a Fowl possessing 

 all four would have a formula A.B.X.Y, the femora-caudal, 

 accessory femoro-caudal, semitendinosus, and accessory semi- 

 tendinosus being all present. On the other hand, a Hawk 

 or Owl, possessing only one, the femoro-caudal, would have 

 as formula A. 



Of these four muscles there are sixteen possible combina- 

 tions; but of these only eight are actually found in birds'^. 



Furthermore, if we denote the presence or absence of the 

 ambiens muscle by the signs + or — , we obviously get six- 

 teen combinations again, though of these only fifteen have, as 

 yet, been observed. For the sake of brevity it will be useful 

 to remember the above four letters, to save the repetition of 

 the full names. To the bearing of these muscles in the classi- 

 fication of birds, I intend to revert at a subsequent part of 

 this paper, only stating here that species of the same genus 

 and nearly allied genera have, nearly in every case, the same 

 mjological formula, and that in families peculiarities involving 

 more than one change are rare, further differences indicating 

 a more remote relationship. 



V. The Distribution of the Deep Plantar Tendons f. 

 In all birds, whatever number of toes they have, there are 

 two deep flexor muscles of the toes, the fleshy bellies of which 

 are situated between the knee and the " ankle,^^ whilst their 

 tendons run along the posterior aspect of the tarso-metatarse. 

 One, arising from behind the external condyle of the femur, 

 is the flexor longus hallucis ; the other, arising from the pos- 

 terior aspect of the tibia and from the fibula, is ihe, flexor 

 perforans digitorum, so called because its terminal tendons 



* P. Z. S. 1874, p. 111. 



t " On the Disposition of the Deep Plantar Tendons in different Birds," 

 P. Z. S. 1875, pp. 339-348. 



