18ft5.] 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. 77S. 



Skeleton. 



Comparison of the skulls in toto (see pp. 310, 311) shows at a 

 glance that the long-billed feature of Mh. 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 measure- 

 ments 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. 



mm. 



Rhea americana 89 



macrorhyncha . 76 



darwini, $ .... 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 niillim. 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, in- 

 stead 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 speci- 

 mens 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. I) in Rh. macrorhyncha. The Cambridge specimens of Rh. 



Proc. ZooL. Soc— 1885, No. XXI. 21 



