OCR Text |
Show 1871.] DR. R. O. CUNNINGHAM ON RHEA. 107 zontal plate of the ethmoid not covered by the frontals and nasals of a different form, being an ellipse with pointed ends instead of a somewhat lozenge-shaped space as in the latter bird. In the former species there is also a much wider unossified space in the interorbital septum between the basi- and presphenoids than there is in the latter ; but this is most probably a difference connected with age. On the other hand the lacuna in the ethmo-alisphenoid plate, immediately beneath the horizontal plate of the ethmoid, is nearly twice as large in Rhea americana as it is in Rhea darwinii. The pterygoid processes of the basisphenoid are also curved more forwards in the former than in the latter. By far the most remarkable distinction in the bones of the skull of the two birds, however, is furnished by the lachrymal. In Rhea americana (Pl. VI. fig 1) the strong process directed backwards (anterior orbital process of authors) is much more elongated than in R. darwinii, and the form of the descending process is also very different. In the former it curves downwards and backwards so as to produce a deeply excavated space on its posterior border, while in the latter (fig. 2) this process is met by another posterior bar of bone so as to connect the space into a large foramen. This will be more readily understood by a reference to the accompanying sketches, in which fig. 1 represents the bone in R. americana, and fig. 2 in R. darwinii, the letter a in both bones indicating the surface of articulation with the cranium. Other minor differences in the bones of the head are probably due to age. Vertebrce.- Except in point of size, the vertebrae of the two species differ but little from one another. I find twenty-one free vertebrae present in R. americana between the head and the lumbosacral portion of the vertebral column; but the axis is wanting in this specimen ; so that the total will be twenty-two should none of the other vetebrse be absent, which does not appear to be the case. As in the specimen of R. darwinii either four or five of the cervical vertebrae have been lost, I cannot speak with absolute certainty as to the number of free vertebrae present; but in all probability it is the same as in R. americana. The styloid processes (pleurapo-physes) of all the cervical vertebrae examined in R. darwinii exist as separate bones, as might, indeed, be expected from the immature condition of the individual. They are fully anchylosed in the third, fourth, and fifth vertebrae (putting the atlas, in which they do not exist, and the axis, which is missing, out of the question) of R. americana, partially so in the sixth, seventh, and eighth, and separate thence to the first rib-bearing vertebra. In both species there are eight rib-bearing vertebrae, the eighth being partially anchylosed to the lumbo-sacral portion of the column. No very obvious differences, save those of size, are to be observed between the vertebral ribs of the two species. In both but three (the third, fourth, and fifth) are connected with the sternum by means of costal ribs. There is likewise no material difference in the form of the sterna of the two birds, allowing for the influence of age. In R. americana the two lateral halves (pleurostea) are united rather firmly together, but would admit of separation without much difficulty ; while in R. darwinii they are separate, partly in consequence |