Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/484

Rh REPTILES [GEOGRAPHICAL viving Mesosuchian Crocodiles of the Jurassic period Gavial-like forms existed, and genera with posterior choanie and proccelous vertebrae (Eusuchians), among them a true Crocodile from the Cambridge Upper Greensand. The western hemisphere exceeded the eastern as regards the abundance of Dinosaurians in this period still more than in the preceding, although it should be remembered that of the latter part of the globe in fact only a small portion has been opened up, and that the little we know of the geology of Central Asia and western China points to the probability of vast palseontological treasures being reserved for the study of future generations. Rhynchoce- phalian genera continue in considerable variety, whilst other small-sized forms are referred to the Lacertilians. The types of Freshwater Turtles which lived in the Jurassic period pass into the Cretaceous. They appear now also in North- American strata : CJielydidas first, ac- companied or immediately followed by Emydidse, a few Trionyx and Chtlonia, and the remarkable Protostega, a precursor of Sphargis. Towards the end of the Cretaceous period its abundant Reptilian life is swept away, and not only every species but almost every Mesozoic genus dis- appears, a wide gap being left in our knowledge of the succession of Reptilian life between this period and the Tertiary era. ertiary. In the lowermost strata of Tertiary age a few isolated remains remind us still of the extinct Mesozoic fauna ; such are one or two genera allied to the Dinosaurians (Agathau- mas) from the Laramic beds of North America. In other respects the Tertiary Reptiles bear already the character of the present fauna ; although families or genera which are distinct from the living predominate in the Eocene, from the Miocene Reptiles gradually approach more and more the specific forms of our time. True Crocodiles (of which one species occurred already in the Cretaceous period), Gavials, Alligators have now entirely superseded the "Mesosuchian" type. The remains of Lacertilians are fragmentary and generally indistinct, so as to offer a wide field for generic distinctions. Anguidae have been distinctly traced back to the Miocene of North America. With a single exception, that of a poisonous Serpent, apparently a Viperct, the remains of which have been found in Miocene deposits at Sansan (south of France), Snakes do not appear before the Eocene, in England as well as North America ; they are mostly large species, with un- certain affinities, but all innocuous (Palseophis, Dinophis, Boavus, Lithophis). Their remains are scarce, and even those of the later Tertiary formations show a greater resemblance to genera which are now-a-days locally far removed than to those living in the same geographical district. Chelonian remains are numerous throughout, probably on account of the resistance which their solid carapaces offered to destructive agencies. Chelydidsz, Emydidx, and Trionycidx and also Marine Turtles con- tinue to exist in an almost uninterrupted series, and approach so much the modern genera that, for instance, in Trionyx, if specific distinctions existed between Miocene and recent species, they cannot be determined in those parts of their skeleton which have been preserved. A peculiar type intermediate between the Marine Turtles and Emys existed in the English Eocene (Chelone). Land Tortoises, the most specialized type of the order, probably did not appear before the Miocene. A number of smaller forms are referable to the genus Testudo, but gigantic forms existed then as now in isolated localities, and exceeded in size the living, like the Colossochelys of the Shvalik Hills, with a carapace computed to be 20 feet long, and the smaller Macrochelys from the German Molasse. It is extremely singular that the existence of these colossal Chelonians would seem to coincide with the period of the first appearance of the type of Land Tor- toises. Of the first appearance of most of the families of the Qnater recent fauna, and of their distribution in the Quaternary "-iry. period, 1 we know next to nothing. With few exceptions the Reptilian type had dwindled to forms of such small size that their remains, if preserved, escape observation cr are too indistinct for determination. THE DISTRIBUTION OF EXISTING REPTILES OVER THE EARTH'S SURFACE. Serious obstacles present themselves at the present time Geo- to a satisfactory treatment of the geographical distribution graphs of Reptiles. We have mentioned at the end of the ^ ! preceding section that the geological record is much too incomplete to permit a comparison to be drawn between their distribution in the present and next preceding periods, and that we are therefore ignorant as to the modes in which their dispersal was brought about, and unable to account otherwise than hypothetically for the numerous instances of apparently anomalous distribution. And, when we limit ourselves merely to the task of describ- ing the facts of their distribution at the present day, it is necessary for such an attempt that the whple class should have previously undergone a thorough rearrangement in accordance with the present state of science, and that the scattered contributions to the various faunae should have been critically examined and treated by the same uniform method. This is unfortunately not the case with the class of Reptilia. The systems and lists of Lacertilia and Ophidia which formed the bases for the treatises on the geographi- cal distribution of these orders by Schlegel, Giinther, and Wallace are buried under an accumulation of an immense number of additions of very different value, which require a uniform critical revision before they can be used in an inquiry of a general biological scope. 2 In the following notes we have been able to include the majority of the genera of Chelonians and Ophidians, with an approximate estimate of the species of each that are known at present. As regards the former Strauch 3 has done much to clear the way, although the small amount of materials known to him from autopsy interfered much with his critical judgment. Not many additions have been made to this order since the time of the publication of his memoirs. Of the numerous Ophidian genera and species we have eliminated all which are based on trivial distinctions, or are named by authors without special acquaintance with the subject. We have not entered into a full consideration of Lacertilian genera, as this would have been useless at a time when they are undergoing a revision, but have restricted ourselves to the families recently proposed by Boulenger. One of the most important results obtained by this inquiry is that the same arrangement of the so-called primary zoological regions is not applicable to all orders of Reptiles, and that the differences in their distribution are so fundamental that they can be accounted for only on the assumption of the various orders and families having appeared and spread over the globe at very distant periods, when land and water were differently distributed over the surface of the globe. At the end of this section we shall show in a short re'sume^ the mutual relations between the various regions with respect to the several orders of Reptiles. The means of dispersal of Reptiles are very limited. 1 An important memoir on Indian Chelonia is published whilst this article is passing through the press, " Siwalik and Narbada Chelonia, " by B. Lydekker, in Palseontol. Ind., vol. iii. ser. x., 1885. 2 Whilst this article is being written, the Lacertilia are undergoing the much-needed arrangement by Boulenger (as mentioned at p. 439), who also has recently published preliminary notes on the distribution of the families of Lacertilians, which have been of great use to the present writer (Ann. and Mag. Nat. Hist., August 1885, p. 77). 8 " Chelonologische Studieu," and "Die Verbreitung der Schild- kroten iiber den Erdball," in M$moires de PAcad. de St PStersb., 1862 and 1865.