The geographical distribution of animals, with a study of the relations ...

;160 DISTRIBUTION OF EXTINCT ANIMALS. [part ii.groups as Carnivora, Perrissodactyle and Artiodactyle Ungulates,Primates, Chiroptera, Rodents, and Marsupials already wellmarked, but in many of these there is a differentiation intonumerous families and genera of diverse character.It is impossibletherefore to doubt, that many peculiar forms of mammaliamust have lived long anterior to the Eocene period ;but thereis unfortunately a great gap in the record between the Eoceneand Cretaceous beds, and these latter being for the most partmarine continue the gap as regards mammals over an enormouslapse of time. Yet far beyond both these chasms in the UpperOolitic strata, remains of small mammalia have been foundagain, in the Stonesfield slate, a member of the Lower Oolite,other forms appear. Then comes the marine Lias formationwith another huge gap ; but beyond this again in the UpperTrias, the oldest of the secondary formations, mammalian teethhave been discovered inboth England and Germany, and theseare, as nearly as can be ascertained, of the same age as theDromatlurium already noticed, from North America. Theyhave bee:i named Microlestes, and show some resemblance tothose of the West Australian Myrmecobius. In the Ooliticstrata numerous small jawbones have been found, which haveserved to characterise eight genera, all of which are believed tohave been Marsupials, and in some of them a resemblance can betraced to some of the smaller living Australian species.however, are mere indicationsThese,of the number of mammalia thatmust have lived in the secondary period, so long thought to beexclusively " the age of reptiles ;" and the fact that the few yetfound are at all comparable with such specialised forms as stillexist, must convince us, that we shall have to seek far beyondeven the earliest of these remains, for the first appearance of themammalian type of vertebrata.Extinct Birds.Compared with those of mammalia, the remains of birds areexceedingly scarce in Europe and America ;and from the wanderin"habits of so many of this class, they are of much less value