Imágenes de páginas


[ocr errors]
[ocr errors]



species 2. apicalis has the apex of the spines dilated and sculptured on the superior or external surface, indicating the presence of a row of osseous shields covered by epidermis only, extending along the middle dorsal line. In the Trias, two such types have been previously known ; viz., the genus Typothorax Cope, from New Mexico, and Aëtosaurus Fraas from Würtemberg.

The discovery of the Permian form in question is important from various points of view. The discovery confirms again a hypothesis proposed by me, several years previously (NATURALIST, 1885, p. 247, Transac. Amer. Philos. Soc., 1892, p. 24). It presents us with what had been previously wanting, forms ancestral not only to the Triassic Reptilia above referred to, but also ancestral to the order of the Testudinata, which according to Quenstadt and Baur appears first in the Trias. The discovery also brings to light an interesting case of homoplassy, since we have two families in no way allied to each other, the one a Batrachian, and the other a reptile, presenting an identical character, and which is so closely similar in the two, that the carapaces cannot be well distinguished on an external view. Internally, however, the characters differ widely. In the case of the Reptilian family (Otoccelidae) the structure is what one finds in the Testudinata and Pseudosuchia (Typothorax); while in the Batrachian it is constructed by an expansion of characters already known in other Stegocephalia.

For the accompanying illustrations I am indebted to the American Philosophical Society.

PLATE XXI. Otocoelus testudineus Cope, From above x.66.

PLATE XXII. Dissorhophus articulatus Cope, x.82; 1 above ; 2 below; 3 anterior view.-E. D. COPE.

Ameghino on the Evolution of Mammalian Teeth.' - The discoveries of M. Ameghino in Argentina have put him in a position to throw a great deal of light on the evolution of the Mammalia. Several problems which are presented by general Mammalian dentition should be greatly elucidated by his material, and some of those suggested by the Toxodont and Edentate types are within his reach almost to the exclusion of other investigators. He bas already made important contributions to the histories of both these orders, while other problems

1 See l’Evolution des Dents des Mammiferes par Florentino Ameghino. From the Bull. Acad de Ciencias de Cordoba, XIV, p. 381; Buenos Ayres, 1896.

remain open.

In the paper of about 1060 pages now before us, M. Ameghino gives his views on the general subject. It seems that in his work Filogenia, published in 1884, he adopted the view of Gaudry of 1878, (previously barely suggested by others), that complex teeth of Mammalia are produced by the fusion of a number of originally distinct simple teeth ; a view which has been supported by Kükenthal and Röse on embryologic grounds. It had been previously believed that additional cusps are the product of plications of the dental crowns of simple teeth, and in 1873 and later I had constructed on that basis a phylogenetic system of dentition. This, as is well-known, proceeds from the simple to the complex, without the element of fusion entering at any point. The series is, for the upper jaw; the haplodont, triconodont, tritubercular,' (sectorial) quadritubercular, quinque- and sextubercular, and the various lophodont forms; for the lower jaw; haplodont, triconodont, tritubercular, tuberculosectorial, (sectorial), quadritubercular, and the various lophodonts. This succession corresponds with the time order both in North America and Europe, and it is to be supposed that it must, therefore, do so in other parts of the earth, wherever the Mammalia have developed a dentition beyond primitive types.

I have never attempted to bring into this system the Monotrematous Prototheria, and have maintained that they constitute a distinct phylum.' My discovery that the dentition of the Permian Cotylosaurian family of the Pariotichida consists of simple teeth arranged in transverse series, induced me to remarks « that the only question that could arise” as to the hypothesis of dental fusion “is with regard to the Multituberculata.” A fusion of the teeth of the Pariotichidæ could produce molars like those of the Multituberculata; but there is no evidence that such a fusion has ever occurred.

Returning to the Eutherian Mammalia, we observe that Ameghino believes that the complex molars have preceded the simple ones in the order of time, and that the tritubercular molar is the result of a loss of a tubercle of the quadritubercular; the quadritubercular the result of

? Rütimeyer used the term trigonodont for triangular molars, without specifica. tion of the number of tubercles. This word cannot take the place of tritubercular, since the evolution is a question of tubercles, and not of shape. Some tritubercular teeth are quadrangular (Periptychus) and vice versa.

3 See Amer. Journal of Morphology, 1889, p. 146. • Proceeds. Amer. Philos. Society, 1895, 439-444.

5 Primary Factors of Organic Evolution, 1896, p. 334. Amer. NATURALIST, 1896, Plate VIIa, p. 801.

« AnteriorContinuar »