Subclass Coraciiformes. 201 



as well as amongst mammals, some variation, even amongst 

 allied species, in the modification of these muscles ; but this 

 variation only occurs within comparatively narrow limits* 

 In nine genera of Accipitres I have found that the inside or 

 front plantar {flexor perforans digitorum) divides into three 

 tendons at the foot of the tarsus, the three tendons leading 

 to the 2nd, 3rd, and 4th digits respectively ; whilst the out- 

 side or hind plantar {flexor longus hallucis) divides into two 

 tendons at the foot of the tarsus, one leading to the hallux 

 and the other to the 2nd digit. In the Osprey {Pandion 

 haliaetus) this arrangement is modified ; the front plantar is 

 the same as in the nine other genera of Accipitres ; but the 

 hind plantar, instead of dividing into two, divides into four, 

 and leads to each of the four digits* The Scansorial arrange- 

 ment of the deep plantar tendons is very different from this ; 

 the front plantar leads to the 3rd digit only, whilst the hind 

 plantar leads to the 1st, 2nd, and 4th digits. The hetero- 

 dactyle arrangement differs, again, from all these modifi- 

 cations ; the front plantar leads to the two front toes (the 

 3rd and 4th digits), whilst the hind plantar leads to the two 

 hind toes (the 1st and 2nd digits). But all these apparently 

 wide deviations from the common type may be generalized 

 as follows : — The flexor perforans digitorum never leads to 

 the hallux, and the flexor longus hallucis always does, except 

 in those cases where the hallux is absent or so small that it 

 does not receive any tendon from either of the deep plant ars. 

 In the Coraciiformes exactly the opposite is the case. The 

 hallux is always present, and always receives its tendon from 

 the flexor perforans digitorum, and not from the flexor longus 

 hallucis. The two plantars are always more or less coalesced, 

 as they are in the Accipitres, Anseres, &c, but may easily be 

 separated by gently tearing them asunder. In very many 

 cases it is not necessary to do this, inasmuch as the tendon 

 to the hallux branches off from the flexor perforans digitorum 

 before the two plantars coalesce. 



It is scarcely possible that such a remarkable condition of 

 the deep plantar tendons could have been independently 

 acquired by two groups of birds. 



