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Show 1896.] ANATOMY OP THE HOATZIN. 627 birds which, although they are certainly to be placed among the Homalogonatae and have close allies in which the ambiens is normal, are themselves without it. In Nycticorax 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 m y second specimen of Opisthocomus, in which the ambiens ended on the knee. The vestige in Eclectus, as in the second Opisthocomus, consisted of three slips from the fibula to the flexor tendons. This additional evidence appears to m e to strengthen the case for the taxonomic value of the ambiens considerably. While there were known only the rudiments described by m e in Eclectus and Ngcticorax, it might have been open to doubt whether or no these really were vestiges of an ambiens. N o w that there have been found in different individuals of Opisthocomus graded vestiges linking m y 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. O n the other hand, I may mention that although Bissura is a Stork without an ambiens, while other genera of Storks possess it, in two specimens of Bissura 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 exceedingly 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 Batites, Fowls, and in Pterocles. I can add to this a very long list of birds, including Rhynchotus, Chauna and Palamedea, Bcdearica, Psophia, and Pulica. The slip is absent in Opisthocomus ; the only other cases that I remember in which this occurs are Asio otus and Rhytidoceros plicatus. Short flexors from the deep) plantar tendons.-The tendon of the flexor longus hallucis is connected with the tendon of the flexor 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 |
