440 M. Serres on Glyptodon ornatus. 



same medium, I shall select the Fauna of the Tuurtia, so well 

 appreciated by M. d'Archiac. 



" What strikes us at once," says that illustrious geologist*, 

 " in the examination of this Fauna, which is still so imperfectly 

 known, the elements in our possession having only been col- 

 lected at three or four points, is the prodigious development 

 and the almost infinite variety of the type of the Tercbratulee. 



" Of these we have determined and described forty-eight spe- 

 cies. Of this number thirty-four, or nearly three-fourths, are 

 new; and there are, besides, more than twenty equally well- 

 characterized varieties. This genus of itself includes nearly 

 one-fourth of all the species that we know in the Tourtia; and 

 when we consider the small thickness of this bed, and the re- 

 stricted space in which it is deposited, we cannot but be asto- 

 nished that a single organic type, after probably a very short 

 lapse of time, should present in the combination of its forms, 

 or of its dimensions, so manifest a proof of the admirable fe- 

 cundity of nature, that it might lead us to doubt of the reality 

 of the species considered in itself." 



Among fossil Vertebrata, the Glyptodons, which flourished at 

 the period of formation of the Subapennine stage of the Pampas 

 of Buenos Ayres, in which the remains of these animals are 

 found exclusively, reproduce the same fact, although in a much 

 less degree; and hence the necessity of well-characterizing the 

 species, however difficult this may be. In fact, if anatomical 

 investigation applied to creatures which no longer exist presents 

 in some respects an attractive interest, it also sometimes exposes 

 us momentarily to certain zoological confusions, which cannot 

 always be avoided. 



The fragments of several extinct and unknown animals, of the 

 same group and the same size, reach us, collected together with- 

 out order, in the same layer of soil. If each skeleton were 

 complete, nothing would be easier than to bring together these 

 disjointed organs, and to reestablish the same natural relations 

 which formerly united them into a single living whole. The 

 difficulty commences when each of them is represented only by 

 a bone, a different organ. Then the naturalist cannot escape 

 the danger of describing as belonging to the same species organs 

 really pertaining to several, except by falling into another error, 

 that of describing a single species under several names. In this 

 alternation of two causes of error, both difficult to shun, I 

 found myself in the comparisons which I propose now to bring 

 before the Academy. 



In my first note on the Carapace of Glyptodon ornatus I 



* "Rapport sur les fossiles du Tourtia" Mem. de la Soc. Ge'ol. de France, 

 2 e seiie, tome ii. p. 291. 



