PHENOMENA OF REGRESSION AND CONVERGENCE 225 



the kind to a crocodile rather than to a Stegoce- 

 phalus, or another to a Marine Turtle rather than to a 

 gigantic Edentatus. An example taken from recent 

 controversies will show us the possibility of an 

 almost complete convergence. Certain small-sized 

 polygonal dermic plates found by Filhol in the 

 phosphorites of the Department of the Lot, were 

 ascribed by this scholar to portions of the carapaces 

 of Tatus, and the opinion of Filhol was founded 

 not only on the identical aspect of the details of 

 ornamentation, but further, on a histological study 

 of these organs. Now these plates are found in 

 situ on the cranium of a Reptile of the Upper Eocene 

 described long ago by Gervais under the name of 

 Placosaurus, and our contemporary, Leenhardt, has 

 recently described a second cranium of this animal 

 armed with its cephalic shield. There is here an 

 identity of structure really surprising and, it must 

 be owned, rather inexplicable. 



The teeth of fossil mammals, generally so special- 

 ized and so characteristic of each group, present, 

 however, a certain number of cases of almost 

 complete resemblance in structure among animals 

 belonging to very different families and even orders. 

 Such is the case, for example, with the long, thick, 

 and smooth canines of the Ungulates of the genus 

 Lophiodon, which resemble, most deceptively, the 

 canines of the Ursidse ; such is, again, the case of 

 the canines, flattened like sword blades and crenel- 

 lated, of the powerful Felines of the genus Machairo- 

 dus. We have recently observed in a species of 

 Ungulate of the genus Brachyodus of the Oligocene, 

 canines so similar to those of a small Machairodus 



