OE" THE ORIGIN OF GENERA. 55 



4. Among the tortoises, the Testudinidae rapidly extend the 

 ribs into a carapace, which fits closely the marginal bones, while 

 equally early in life the elements of the sternum unite together. 

 This is also the case with most Emydidee ; among whose genera, 

 however, we find the transitional scale. In Dermatemys and 

 Batagur the carapace is very late in attaining its complete ossifi- 

 cation, while the plastron is early finished. In Chelydra, on the 

 other hand, while the carapace is even more slowly developed, the 

 plastron is never free from its larval fontanelles. In the marine 

 turtles neither plastron nor carapace is ever completed, while 

 in the Trionychidag the marginal bones are also entirely unde- 

 veloped. 



In order that this last illustration be a true one for the theory 

 in question, as applied to ihQ families, these developmental char- 

 acters should be the true distinctive features of these families re- 

 spectively. This, as is well known, they are not. The Chelo- 

 niidse are characterized by the form of their anterior limbs, which 

 is in an adaptive structure, while the Testudinidae similarly are 

 distinguished by an extreme opposite modification of foot-struct- 

 ure, adapted to an extreme difference of habit. Here there is an 

 example of the co-working of both laws. Nevertheless, we only 

 claim at present to show the developmental relation of genera of 

 the same family and the same series. This we see among the 

 Emydidae. 



5. In the important character of the sciitellation of the tarsi 

 among the Passerine birds, the ^^boot" appears early in life in the 

 highest Oscines, later in the lower, and does not ai:)pear at all in 

 the majority. In respect to the still more important feature of 

 the long posterior plates which appear very early in most Oscines, 

 in the Myiadestes type * they appear late, the squamse remaining 

 long, while the Clamatores never develop the plates, not advancing 

 beyond the infantile squamous stage. 



6. It has been shown by Falconer that the genera of great 

 Proboscidians form a remarkably regular and graded series, dis- 

 tinguished by their dentition. These are Dinotherium Kaup, 

 Trilophodon Falc, Mastodon Cuv., Pentalophodon Falc, Stego- 

 don Falc, Loxodon F. Cuv., and Elephas Linn. In the first 

 there are but two cross crests on the third molars, and a pair of 

 permanent mandibular tusks ; in the second, three cross crests 



* Baird, " Review of Birds of North America." 



