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Show 314 DR. H. GADOW ON THE SPECIES OF RHEA. [Mar. 17, Makes 33 vertebrae between the occiput and the primitive inter-sacral nerve, which in all the three species is the 34th spinal nerve, no matter whether being at the same time the last ischiadic root, without connection with the pubic plexus as in Rh. macrorhyncha, or being in connection with the ischiadic plexus by a ramus communicating only and belonging chiefly to the pubic region, as in Rh. darwini 2 • W e have therefore to compare the series of the first 15 vertebrae of Rh. macrorhyncha with the first 14 vertebrae of the other two species. There can be no doubt now, that the greater number of neck-vertebrae is not produced simply by a shortening of the first pseudo-thoracic rib, as it generally happens in apparently similar cases of variation in the number of ribs, but, on the contrary, that the whole of the thoracico-lumbar region has been affected by a tail-ward " shifting " to the extent of one metamere ; and it is also absolutely certain that the serial differences between Rh. americana and Rh. macrorhyncha have not been brought about by the inter- or ex-calation of a vertebra. In order to determine whether Rh. macrorhyncha possesses a proportionately longer neck than the other species, we have to compare the length of the neck to some distance which must stand in some reasonably conceivable correlation to the former. The distance between the neck and the acetabular region appears to be rather advisable for this purpose. Again, one may fairly well suppose that in essentially terrestrial birds there might be a correlation between the length of the neck and the length of the hind limbs. However, the elevation of the trunk above the ground depends not only upon the length of the hind limbs, but also upon the angles formed by various segments of the limb to each other and to the pelvis. These considerations therefore leave the following results open to doubt. At any rate I have made the calculations, in which a certain limit of error in measuring the neck and limb must be allowed. R. macro- R. darwini <j>. R. americana. rhyncha. centim. centim. centim. Length of neck from atlas to cephalic end of transitional (14th or 15th) vertebra.'. 527 57-0 47*5 Length of femur -f tibia -f-tarso- metatarsus 88*3 87*4 73*1 If the neck of Rh. darwini were of the same proportionate size as that of Rh. macrorhyncha, there ought to be 52*7 : 88*3 = 47*5:73*1 but 52*7x73*1=3852*37 88*3x47*5 = 4194*25 |