1896.] ANATOMY OF THE HOATZIN. 627 
birds which, although they are certainly to be placed among the 
Homalogonate and have close allies in which the ambzens is normal, 
are themselves without it. In Nycticoraa gardeni the ambiens is 
absent; but in two specimens that I have dissected I found a slip 
to the flexor muscles from the fibula, similar to that in the 
Opisthocomus here figured, although it arose rather lower down the 
fibula. In Eclectus roratus, which again is devoid of an ambiens, 
although many Parrots are provided with it, I found a vestige 
almost precisely similar to that present in my second specimen of 
Opisthocomus, in which the ambiens ended on the knee. The 
vestige in Helectus, as in the second Opisthocomus, consisted of 
three slips from the fibula to the flexor tendons. This additional 
evidence appears to me to strengthen the case for the taxonomic 
value of the ambiens considerably. While there were known 
only the rudiments described by me in Eelectus and Nycticoraw, 
it might have been open to doubt whether or no these really were 
vestiges of an ambiens. Now that there have been found in different 
individuals of Opisthocomus graded vestiges linking my rudimentary 
condition with a complete ambiens, there seems no room to doubt 
that some, at least, of the Homalogonatous birds devoid of an 
ambiens have once possessed it. On the other hand, I may mention 
that although Dissura is a Stork without an ambiens, while other 
genera of Storks possess it, in two specimens of Dissura episcopus 
I have recently dissected I could find no trace of the vestige. 
Apart from possible systematic value, it is of interest to find 
variations of so great magnitude in a few specimens of a bird. 
Prof. Weldon has recently shown, after examination of an ex- 
ceedingly large number of individuals of the shore-crabs, that very 
slight deviations may be associated with a larger death-rate. In 
the case of creatures so difficult to shoot as is Opisthocomus it may 
be the case that those actually examined have, from the greater 
magnitude of their variations, been less able to escape. 
In conclusion, I may place on record three minor points in the 
myology of Opisthocomus, the only remaining features which seemed 
to me worth recording at the present time. 
Tendons of the perforated and of the perforated and perforating 
flexors of the third digit.—In most birds the tendons of these are 
connected by a short stout vinculum immediately before they 
reach the foot. Gadow mentions that this occurs in Ratites, 
Fowls, and in Pterocles. Ican add to this a very long list of birds, 
including Rhynchotus, Chauna and Palamedea, Balearica, Psophia, 
and Fulica. The slip is absent in Opisthocomus ; the only other 
cases that I remember in which this occurs are Asto otus and 
Rhytidoceros plicatus. 
Short flecors from the deep plantar tendons.—The tendon of the 
flexor longus hallucis is connected with the tendon of the flewor 
perforans by a strong vinculum and then supplies the thumb. 
A strong muscular slip, certainly absent in most birds, leaves the 
longus hallucis tendon immediately distal to the vinculum and runs 
to the fourth digit. A similar muscular slip leaves the tendon of 
