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Stem Reptiles. In the field of the oldest reptiles, those of the Carboniferous and Permian, perhaps the most important con- tributions are those by S. W. Williston and D. M. S. Watson. The former, in his paper on The Phylogeny and Classification of the Reptiles (1917), traces the rise of the common amphibian- reptilian stock through the " Protopoda," which are so far known only from certain footprints of Upper Devonian age. According to Williston, who built on Osborn's system of 1903-4, the primi- tive reptilian stock early divided into the following series:

Anapsida (Cotylosauria and their specialized descendants, the modern tortoises and turtles).

Synapsida (Theromorpha or pelycosaurs, etc. ; Therapsida, or mammal-like reptiles, the latter giving rise to the mammals ; plesio- saurs).

Diapsida (reptiles with two temporal arches, such as crocodiles, dinosaurs, rhynchocephalians; this stock gave rise to birds).

Parapsida (including the proganosaurs, ichthyosaurs, lizards, mosasaurs, snakes).

Watson (1917), in his Sketch Classification of the Pre-Jurassic Tetrapod Vertebrates, assigns a high value in classification to the characters of the brain-case. A general and conservative classifi- cation of the early reptiles is given by W. K. Gregory (1920). The most primitive known reptile, Seymouria, from the Permo- Carboniferous of Texas, almost bridges the gap between the Amphibia and the Reptilia. Watson (1919) gives a morphological description of this reptile, accompanied by valuable figures and reconstructions of the skull and skeleton.

The habits and environments of the teeming reptilian and amphibian faunas of the Permo-Carboniferous of North America are intensively considered in a memoir by E. C. Case (1919), which also deals with the stratigraphy, climatology and palaeogeog- raphy of the late Palaeozoic.

Mammal-like Reptiles. In no other field of fossil reptiles has the progress of discovery been more satisfactory than in that of the mammal-like reptiles of South Africa, as set forth in numer- ous papers, especially by Watson (1913-4), Haughton (1918), Broom (1913-4), van Hoepen and others. The relationships of these animals with other reptiles and with the mammals have been reviewed by W. K. Gregory (1920-1).

Marine Reptiles. These have always been of great interest on account of their secondary adaptations to aquatic life which have been ably discussed by Abel (1912, 1919). One of the outstanding contributions of new material in this field is the British Museum Catalogue of Marine Reptiles of the Oxford Clay by C. W. Andrews (1910-3). The origin and relationships of the plesiosaurs and their allies are treated by von Huene (1921).

Dinosaurs. The Triassic dinosaurs of Europe are of particular interest because some of them tend to connect the very diverse carnivorous and herbivorous saurischian dinosaurs of later ages. Here the leading author is F. von Huene, in a long series of papers and memoirs. Plateosaurus, perhaps the most primitive of these reptiles, has been fully described both by von Huene and by Jaekel (1913-6). Primitive dinosaurs from the summit of the Karroo series in South Africa (Gryponyx, Massospondylus, etc.) are described by Broom and Haughton. During the long ages of the Jurassic the gigantic sauropodous dinosaurs attained their maximum in size and specialization. The leading feature in this field is the description of the strange and monstrous dinosaurs of the Tendaguru fauna of East Africa in the collections of the Berlin Museum, by Janensch (1914). One of the most remarkable of the North American sauropods is the genus Camarasaurus, which has been intensively described by Osbornand Mook (1921). Barosaurus, a gigantic relative of Diplodocus, with a tremendously heavy neck, has been described by R. S. Lull (1919). Tyranno- saurus, the greatest carnivorous reptile of all time, and Struthi- omimus, a contemporaneous ostrich-like dinosaur, have been described by Osborn (1912-9). The highly varied and grotesque armoured dinosaurs, namely, the Ceratopsia and related groups, have been the subject of numerous papers by Gilmore, Brown, Lambe and others.

Pterosaurs. The pterosaurs, or flying reptiles, have continued to excite the interest of students of flight, such as Abel (1912), Watson and Hankin (1914), Arthaber (1921). The greatest

flying reptile, Pteranodon, is the subject of a memoir by Eaton (1910) of Yale University.

Chelonians. An important memoir by O. Jaekel (1913-6) describes the skull, skeleton, carapace and plastron of Triasso- chelys dux from the Upper Triassic of Germany. Although widely differentiated from all other orders this reptile was the most primitive of all known chelonians. Of even greater interest is the Eunotosaurus from the Permian of South Africa which Watson (1914) describes as a veritable " Archichelone."

Fossil Birds. Dialryma, a gigantic ground bird from the Lower Eocene of Wyoming, has been described by W. D. Matthew and W. Granger (1917) from a nearly complete skeleton, which is a most rare and valuable fossil. This bird, which has no near relatives, was about seven feet high and of massive proportions, with an enormous head and great compressed beak. The wings were vestigial. This high degree of specialization at such an early epoch indicates that the modernized groups of birds were differ- 'entiated during the latter part of the Age of Reptiles.

Monographs on Special Groups of Tertiary Mammals. The fossil mammals of the basal and Lower Eocene of the western United States are represented in the American Museum of Natural History by collections numbering many thousands of specimens which are being described jointly by Matthew and Granger (1915). Besides describing many new or little known forms these authors also deal with the relationships and mor- phology of the various groups of early placental mammals. In the paper dealing with the edentates and their relatives, the " palaeanodonts," Matthew (1918) advances the view that the modern Pholidota (Pangolins) are an offshoot of the primitive " palaeanodonts " of the Lower Eocene. Other papers of the same series cover the Creodonts, Insectivores, Primates and Condylarths.

Several mid-Tertiary mammalian groups, such as chalicotheres, entelodonts and the diceratheres, have been revised in the publica- tions of the Carnegie Museum, Pittsburgh, by W. J. Holland and by O. A. Peterson.

Baluchitherium, perhaps the most gigantic land mammal of all time, has been described by C. Forster-Cooper (1913) from a huge atlas, astragalus, cervical vertebrae and limb bones from the Upper Oligocene deposits of the Bugti Hills of Baluchistan.

The evolution of the Sirenia is treated by Abel (1921) and by Deperet (1920); that of the Cetacea by Abel (1919) and by Winge (1918-21). The phylogeny and evolution of the Pro- boscidea are considered in the researches by Schlesinger (^917), Matsumoto (1915) and Osborn (1918-21). The Eocene and Oligocene titanotheres have been dealt with in numerous papers by Osborn in preparation for his monograph on these extinct animals. The revision of the mid-Tertiary Equidae by Osborn (1918) affords an exceptionally full document on the exact course of evolution in the multitudinous phyla of a typical mammalian family. A most valuable expansion of our knowledge of the anthropoid apes of the mid-Tertiary is found in the work of Pilgrim (1915) on the fossil apes of India of the genera Dryopi- thecus and Sivapithecus.

South American Fossil Mammals. The strange offshoots of the ungulate and edentate orders which swarmed in Patagonia during the mid-Tertiary and Pleistocene times are treated in the excellent memoirs of the Princeton University Patagonian expedi- tions by W. B. Scott. Herluf Winge has admirably monographed the fossil and recent edentates of Brazil. The mammalian fauna of the Deseado formations is described by F. B. Loomis of Am- herst College. These and other investigations are correcting the erroneous correlations by Ameghino, in which the older mammal- bearing horizons of Patagonia were assigned to the Cretaceous. This more modern work indicates that the Pyrotherium beds are not older than Upper Eocene and that the Santa Cruz formation is of Lower Miocene Age.

The Pleistocene fauna of Tarija, Bolivia, is the subject of a beautiful memoir by Boule and Thevenin (1920), in which the anatomy and relationships of " Mastodon " andium and of the highly specialized horses Hippidium and Onohippidium are treated.