246 MR. W. P. PYCRAFT ON THE MORPHOLOGY AND 



male R. darivini the ambiens of the left thigh stopped at the knee, as in the female, 

 but in the right thigh the muscle was typically developed and passed the knee with a 

 strong tendon. 



In Casuarius the ambiens, according to Garrod, is wanting. Concerning this, 

 Dr. Gadow, in describing the ambiens of Rhea, a portion of which we have just 

 quoted, continues, " This abnormal condition of m. ambiens, which seems to be pre- 

 vailing in Darwin's Rhea, is the intermediate stage between a typically developed 

 m. ambiens and such forms in which, as in Casuarius, this muscle has lost still more 

 of its independence, and then only forms an additional head of the median part of 

 the portio media m. femori tibialis s. vasti. Without an elaborate examination 

 and comparison of the formation of these muscles with their nerve-supply, 

 we should, with Garrod, come to the conclusion that Casuarius did not possess an 

 ambiens muscle. The assumption of still further reduction of the distal portion of 

 the m. ambiens explains what I have observed in some Passerine birds, e. g., in a 

 specimen of Lanius bentet, in which the m. femori tibialis internus, besides being 

 strongly developed, received in its proximal part a thin spindle-shaped semitendinous 

 head from the pubic spine. This additional little slip is probably the last trace of 

 the ambiens muscle, which is now generally lost by the Passerine birds. 



" The case above described is one way in which this muscle gets lost ; in other cases, 

 e. g., Ciconia and Phoenicopterus, the reduction does not begin by its tendon becoming 

 attached to the neighbouring tendons in the knee-region, but the whole muscle shows 

 a diminution of its tendon and belly to a mere thread, till at last this also disappears, 

 e. g., in Abdimia and Xenorhynchus." 



The muscle-formulae of the Pakeognathce is as follows : — 



A.B.X.Y +. 



Struthio 



Apteryx 



Casuarius 



Crypturi 



Rhea B . X . Y + . 



Dromeeus B . X . Y — . 



The deep flexors of the foot of the Struthiones belong to the types IT. and IV. of 

 Gadow. 



Apteryx alone represents type II. The vinculum is strong, and runs directly 

 downward to join the flexor proj c undus. The flexor hallucis tendon is slender. 



Struthio, Rhea, Casaurius, and Dromwus all belong to type IV. The separate 

 tendons of the flexor hallucis longus and of the fl. perforans s. profundus can 

 be distinguished as far as the middle of the tarso-metatarse, when they completely 

 fuse. 



Flexor brevis digiti in., which arises from the tendon of the flexor profundus and is 

 inserted into digit in., occurs only in Rhea amongst the Struthiones. 



