224 PUZZLES IN PALiEONTOLOGY. 



ised that their origin must be sought for in paleozoic times. 

 Unnumbered ages ago —before the Cretaceous and OoUtic strata 

 were laid down — there were already placental and implacental 

 mammals. The former, which Prof. Marsh proposes to call Pauto- 

 t/ien'a, possessed, amongst others, the following characteristics : — 

 their teeth equalled or exceeded the normal number, some types 

 possessing sixty teeth. Their pre-molars and molars were 

 imperfectly differentiated, without or with hardly any diastemas, 

 and possessed numerous cusps of the insectivorous type ; the 

 canines sometimes well marked and trenchant, sometimes differing 

 little from the molars. The jaws were usually very long, as 

 became animals which could boast of sixty teeth. But even then 

 specialisation had had time to work effectually ; the genus Faitro- 

 dontidcB had short and massive jaws, with but six pre-molars and 

 molars on each side ; yet the character of the dentition is the same 

 as in the many-toothed forms. The Pantotheria were probably all 

 insectivorous, with such vegetable additions as the insectivora will 

 still feed upon. 



The second order to which all the other American Jurassic 

 mammals but one are referred by Professor Marsh, is called by 

 him " AUotheria." Their characteristics are strikingly different to 

 those of the Pantotheria ; their teeth are much heloio the normal 

 number, and the compressed pre-molars are marsupial in type ; 

 the canine teeth are wanting ; the pre-molars and molars are 

 specialised. They were probably a sub-order of marsupials, 

 somewhat resembling the kangaroo rats. Some were of minute 

 size ; the largest was about the size of a rat. 



Of the third group of Jurassic mammals, but one lower jaw 

 has been found, but this is so characteristic, and so different from 

 other forms, that it has been ])laced in a distinct group by Profes- 

 sor Marsh, under the name of Diplocy?iodo)itid(B. 



Surely, the puzzle of evolution, from the pala^ontological point 

 of view, has been made more instead of less intricate by these 

 last discoveries. We now know that the two great lines of 

 l)lacental and implacental mammals have come down side by side 

 for some millions of years, and when we first find their remains the 

 si)ecics are so highly specialised, and so much differentiated, that 

 we know their first beginnings must be put indefinitely further back. 



