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Show 1885.] DR. H. GADOW ON THE SPECIES OF RHEA. 309 FORBES, W. A. Eleven weeks in North-eastern Brazil. Ibis, 1881, p. 312. . On the Conformation of the Thoracic End of the Trachea in the " Ratite " Birds. P. Z. S. 1881, p. 778. Skeleton. Comparison of the skulls in toto (see pp. 310, 311) shows at a glance that the long-billed feature of Rh. macrorhyncha is less due to a larger or stronger bill than to the narrower, more slender shape of the whole skull. In order to explain this, the following measurements have been taken :- ad. The length of the mandible from the hindermost upper angle of the os angulare to the tip of the os dentale. oe. Direct distance between the middle of the occipital region (on the outer surface of the skull) to the anterior end of the ethmoidal region, at the point *. ep. Distance from * to the tip of the os premaxillare. jug. Greatest outer distance between the two jugal arches. oe. ep. mm. 91 76 71 ad. mm. 162 140 135 JU9- mm, 75 55 mm. Rhea americana 89 macrorhyncha . 76 darwini, 2 .... 80 This table shows that the distance ep (corresponding fairly with the length of the bill) in Rh. macrorhyncha equals that of oe, whilst in Rh. americana it is rather longer, and in Rh. darwini considerably shorter. The latter species has therefore the proportionately shortest, Rh. americana the longest " bill." However, the name of Rh. macrorhyncha is less unjustifiable if we consider the width between the two jugular arches at the point of their greatest distance, the latter being 75 millim. in Rh. americana, and only 55 millim. in Rh. macrorhyncha ; whilst in proportion to the total length of the skull, it should measure 65, or in proportion to the distance ep 63, instead of 55 millim. Owing to this formation the skull of Rh. macrorhyncha has a much more slender appearance. According to Cunningham, the lachrymal bones constitute an important distinctive character between the Common and Darwin's Rhea. In his specimen of the Common Rhea, the descending anterior orbital process of the lachrymal bones was deeply notched, whilst in Rh. darwini this notch was converted into a large foramen by another bar of bone, for the reception of one of the orbito-nasal air-sacs. I found, however, this foramen besides in the two specimens of Rh. darwini, likewise in the adult Rh. americana, but a deep lateral notch (like that figured by Cunningham, P. Z. S. 1871, pi. vi. fig. 1) in Rh. macrorhyncha. The Cambridge specimens of Rh. PROC. ZOOL. Soc -1885, No. XXI. 21 |
