285 



a right angle with the scapula. Coracoids not expanded at the distal ends. Humerus with the 

 radial and ulnar processes diverging. Digits usually elongated and the median ones furnisht 

 with three phalanges; only in Terrapene are the median anterior digits reduced to 2 phalanges. 

 Feet fitted primarily for walking, but nearly always provided with a web between the toes. 

 Skull without temporal roof, except the zygomatic and postorbital arches, the former 

 occasionally wanting. Stapedial passage open. Neck completely retractile within the shell. 



In the present work the writer, following by conviction the example of a large number of 

 students of herpetology, the names of whom are mentioned by Louis Agassiz in his Contri- 

 butions to the Natural History of the United States (vol. I, p. 356), has separated the true land- 

 tortoises, the Testudinidae, from the more or less aquatic forms that are to be known as the 

 Emydidae. For this course there appears to be found in the structures of the two groups 

 abundant justification. Furthermore, the two families have been clearly separated ever 

 since the time of the Lower Eocene. 



Even as thus restricted the Emydidae contain more genera and species than any other family 

 of Testudines. According to Mr. G. A. Boulenger's Catalogue of Chelonians, the Emy- 

 didae, excluding the land-tortoises, embrace seventeen genera and sixty-four species; and 

 these numbers are to be increast, rather than diminisht. Probably no other family displays 

 a greater range of habits. Some of the species are almost wholly aquatic, as those of the 

 genera Chrysemys and Graptemys; others are semiaquatic, as the members of Emys and 

 Clemmys; while still others, as Terrapene, are nearly as terrestrial as the Testudinidae them- 

 selves. Of the aquatic forms a few, as the diamond-back terrapins (Malaclemys) affect the 

 salt waters of the sea-coasts; while the greater number inhabit fresh-water lakes and streams. 

 Most of the species are carnivorous in their diet, but a few, as the box-tortoises (Terrapene) 

 subsist on vegetation. 



As regards their present-day distribution, the Emydidae are found occupying portions of 

 all the continents and the larger islands of the warmer portions of the globe, except Australia 

 and the neighboring islands (fig. 13, page 33). It is evident that by some ancient connection 

 of Australia with other lands, probably with Asia, the Pleurodires were enabled to occupy 

 that southern continent, as well as New Guinea; but this connection was interrupted before 

 the Emydidae and the Testudinidae had passed over that bridge. 



Altho the Emydidae have secured a foothold in the African continent, their distribution is 

 limited to the northwestern and northern coasts. Only 2 species are known from Africa 

 Emys orbicularis and Clemmys leprosa. The first is widely spread over Europe and south- 

 western Asia and is found in Algeria. It is within the range of possibility that it has 

 been introduced into Algeria through human agency. Clemmys leprosa is found along the 

 southern Mediterranean coast, from Tunis to the Atlantic, thence southward to Senegal. 

 It has evidently entered Africa at a late geological period and has prest southward along 

 the Atlantic coast until it has reacht the border of the Ethiopian region. To turtles, as to 

 other groups of animals, the Sahara desert has for a long period stood as a barrier to their 

 southern migration. 



It is a fact difficult, perhaps impossible, of explanation at present, that while the Emydidae 

 are absent from the Ethiopian region, the Testudinidae, a strictly terrestrial family, and the 

 Trionychidae, wholly aquatic in their habits, abound in that great region. It might seem that 

 where the Trionychidae could go the Emydidae could easily follow. However, the Trionychidae 

 are a much older family and might have entered Africa before the Emydidae had become 

 widely distributed and at a time when the connections of the continents were greatly different 

 from those now existing. It seems establisht that some of the species of Trionychidae have been 

 able to adapt themselves to life in sea-water, and this ability may have allowed them to work 

 their way along sea-coasts from one fresh-water region to another. 



The Testudinidae, on the other hand, are less dependent on abundance of water for exist- 

 ence than are the Emydidae and might have made their way across the Sahara or the deserts of 

 western Asia into the Ethiopian region. To the writer it appears more probable that the land- 

 tortoises reacht Africa from India by a land connection, of which the Seychelles, Mauritius, the 

 Aldabras, and Madagascar are remnants which within recent or Pleistocene times have been 

 inhabited by gigantic Testudinidae. 



