NATURAL SCIENCES OP PHILADELPHIA, 251 



should be the true distinctive features of these families respectively. This, 

 as is well known, they are not. The CheloniidEe are characterized by the 

 form of their anterior limbs, which is in an adapted structure, while the Tes- 

 tudinidse similarly are distinguished by an extreme opposite modification of 

 foot-structure, adapted to an extreme difference of habit. Here there is an 

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

 sent to show the developmental relation oi genera of the same family and the 

 same series. This we see among the Emydid;c. 



5. In the important character of the scutcllation of the tarsi among the 

 Passerine birds, the "boot" appears early in life in the highest Oscines, later 

 iu the lower, and does not appear at all in the majority. lu 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, th ' squamaa 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, distinguished by their denti- 

 tion. These are Dinotherium Kaup, Trilophodon Falc.,f Mastodon Cuv., 

 Pentalophodon Falc, Stcgodon 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 and man- 

 dibular tusks only permanent in some males ; in the third, four cross crests 

 and the mandibular tusks all deciduous; in the fourth, five cross crests 

 on the third molar; tusks unknown. In Stegodon the tusks cease to ap- 

 pear, the crests of the third molar become more numerous, and embrace 

 between them, in the bottoms of the valleys, a strong deposit of cementum. 

 In Loxodon the crests have the whole interspaces filled with cementum, while 

 the same thing holds in Elephas, with a greatly increased number of cross 

 crests, which beconfe vertical laminae. The laminar character has become 

 apparent from its rudimental condition in Stegodon. 



Now these are stages of development, though not in a continuous, single line. 

 The shedding of the inferior tusks takes place earlier and earlier in the genera 

 from Dinotherium, till they never appear in Stegodon. The molar teeth, it is 

 well known, present, as they succeed each other from back to front, a regu- 

 larlj^ increasing number of transverse crests in the same species. Thus, in 

 Trilophodon ohioticus the first molar presents but two, while the last 

 presents six. The last molars of other genera present a very much increased 

 number. What is it then, but that the increased number of crests in the third 

 molar, definitive of these genera, is an acceleration of growth ; the fourth 

 in Trilophodon is structurally third in Mastodon, and the fourth of Mas- 

 todon being third in Pentalophodon, the fourth of Pentalophodon becoming 

 third in Stegodon, and so to the end ? This is confirmed from the proven fact of 

 the disappearance of the premolars. They are fewer in Trilophodon| than in 



♦B.aird, Review Birds N. America. 



tThe genus Mastodon, as left by Cuvier, embraced two genera, as has been clearly shown 

 by that excellent pala?antologist, the late Dr. Falconer. He named these genera Trilopho- 

 don and Tetralophodon. It appears to us that this was unnecessary, as he was aware 

 that Dr. Godm.an had named the American Mastodon Tetraeaulodon from its sometimes 

 persistent inferior tusks, a rharacter distinguishing it from the Liter genera of the series 

 though not so trenchantly as the three crests of its third molar, as pointed out by Fal'. 

 coner. As this group was taken from the Cuvierian Mastodon, it should retain Godinan's 

 name, for the T. ohioticus, the T. a n g u s t i d e n s and T. h u m b o I d i i, while Cuvier's 

 name should be preserved for the remainder, viz., M. 1 o n g 1 r o s t r i .s, M. b o r s o n i, M. 

 a r V e rn e n s 1 s, etc., the Tilralophodtms of Falconer. 



JThe two-crested and first three-crested molars are usually called milk molars, because 

 early shed. As, however, they are not succeeded by any subsequent teeth, but are simil.ir 

 to those which lie behind them in the jaw, I cannot see why they are not true premolars. 

 Dr. Warren, in his monograph of Mastodon ohioticus, says " This is called the third 

 deciiluous tooth, but why it ia more entitled to this epithet than the two which follow it 

 would be difficult to determine. Are not the first and seeond .so-called permanent teeth 

 equally deeiduous, since they are shed, and leave the last permanent molar solitary '" 

 V. p. 6!). Flower says (Transao. Royal Society, 1867. C38), '-.In the Dugong and in Iheex- 

 isting Elephants the successional process is limited to the incisor teeth." 



1868.] 



