To the earlier, probably middle, Miocene may be referred the wonderful Santa Cruz fauna of Patagonia. It is extremely difficult to convey to the reader any adequate conception of this great assemblage of mammals, because most of them belonged to orders which have altogether vanished from the earth and are only remotely like the forms with which we are familiar in the northern hemisphere. To one who knows only these northern animals, it seems like entering another world when he begins the study of the Santa Cruz fossils. If any North American mammals had then entered South America, which is not probable, they had not extended their range as far as Patagonia. Marvellously rich and varied as the Santa Cruz fauna was, it did not contain everything that we should expect to find in it; several recent families of undoubtedly indigenous South American origin have left no ancestors in the early Miocene formations. For this, there are several obvious reasons. In part, these gaps in the history are merely due to the accidents of collecting and some of them will almost certainly be filled by future exploration. Other absentees will probably never be found, because the Santa Cruz beds are known only in the very far south, and the Miocene climate of the region, though much milder and more genial than the present one, must have been unsuitable for many tropical animals. Again, the Patagonia of that time appears to have been a country of open plains, with few trees, and hence arboreal forms were rare. [Illustration: FIG. 132.—Diagram to illustrate the comparative sizes of the Santa Cruz mammals, a modern pointer dog, within the rectangle, to give the scale. 1. _†Cladosictis lustratus_, predaceous marsupial. 2. _†Protypotherium australe_, †typothere. 3. _†Eocardia excavata_, rodent. 4. _†Stegotherium tesselatum_, armadillo. 5. _†Propalæohoplophorus australis_, †glyptodont. 6. _†Hapalops longiceps_, †ground-sloth. 7. _†Thoatherium minusculum_, †litoptern. 8. _†Astrapotherium magnum_, †astrapothere. 9. _†Prothylacynus patagonicus_, predaceous marsupial. 10. _†Theosodon garrettorum_, †litoptern. 11. _†Nesodon imbricatus_, †toxodont.] While great numbers of large, flightless birds, some of them of enormous size, were entombed in the volcanic ash and dust which were spread over such wide areas and to such great depths, the extreme scarcity of reptiles is surprising; a few remains of lizards have been found, but no snakes, crocodiles, or tortoises, and we have no information as to the plant-life of the region at that time. The mammals were almost all of small or moderate size; only one or two species were really large.