November 1, 1892.] 



KNOWLEDGE 



215 



(into the consideration of which it woidd be difficult to 

 enter here), it will be apparent that the view of regarding ] 

 Plaffiiiula.r as a new allj" of the rat-Iiangaroo was the result 

 of attaching too much importance to resemblances, and 

 overlooking diiierences. Indeed, such resemblances as do 

 exist between the two may be regarded merely as a well- 

 marked instance of the phenomenon known as pnnillilisin, | 

 which has only lately received at the hands of zoologists j 

 the amount of attention it merits. By parallelism, we 

 may explain, is meant a more or less marked resemblance 

 between homologous organs or parts, which has been 

 acquired independently, and is not the result of direct 

 inheritance. An excellent instance of parallelism is 

 afforded by the development of cannon-bones in the horse 

 and the ox, such cannon-bone consisting in the one case 

 of a single metacarpal element, and in the other of two 

 such metacarpals fitted together. Taking it, then, as 

 proved that Fhi<tiaul(ix is not a near ally of the rat- 

 kangaroo, we have to consider whether it can be affiliated 

 to any other group of existing mammals. Before doing so, 

 we have, however, to mention that there are certain other 

 secondary mammals allied to Plaiiiaulax, in which the 



whole of the cheek-teeth 

 are like the true molars of 

 the latter. We have already 

 stated that in riwiiaida.i- 

 the lower molars had a 

 median longitudinal 

 groove, and it may be added 

 that the ridges bordering 

 such grooves are sur- 

 mounted by a number of 

 small tubercles. In the 

 upper jaw, if we may judge 

 by some allied genera, the 

 molars had three such 

 tuberculated ridges, sepa- 

 rated by two grooves. 

 Similar molars occur in 

 the skull represented in 

 Fig. 6, which is that of a 

 ma.mmal discovered a few 

 years ago in the secondary 

 rocks of South Africa, and 

 n&med by Sir E. Owen Tritylodon ; but it will be noticed 

 that there is no trace of the cutting and obliquely -grooved 

 premolar teeth of I'ldi/iaulux, the premolars being like the 

 molars. Detached molars of similar type have been found (as 

 mentioned in our article on " Egg-laying Mammals ") in the 

 trias of Stuttgart, and others occur in the Stonesfield slate. 

 Moreover, in Dorsetshire and North America there are 

 certain nearly allied mammals (Bolodon) in which the 

 upper molars have only two, in place of three, longitudinal 

 ridges of tubercles. These forms, if other proofs were 

 wanting, clearly show, indeed, that the resemblance 

 between I'huiiaulax and the rat-kangaroo is not a genetic 

 one. When, however, the molar teeth of the type in 

 which there are but two longitudinal rows of tubercles are 

 compared with the transitory teeth of the Australian 

 duckbill (see article on " Egg-laying Mammals "), a certain 

 resemblance can be detected between the two, which 

 seems sufficient to indicate (as mentioned in that article) 

 that in PlaijiaKhi.r and Tritylodon we have to do in all 

 probability with ancient types of egg-laying mammals. 



Till within the last few years the cretaceous period formed 

 a complete gap as regards the history of mammals ; and 

 seeing that in Europe, with the exception of the wealden, 

 the rocks of this system are mainly of marine origin, while 

 some of them, like tlie chalk, were laid down in seas of 



Fig. 6. — Uudei- part ui the skull 

 of a South African Secoudarv 

 Mammal ; two-thirds natural size. 



considerable depth, this absence of mammalian remains is 

 not to be wondered at. In the United States the condition 

 of things is, however, very different. There the upper- 

 most cretaceous rocks are of fresh-water origin, and 

 constitute a series known as the Laramie, which is in 

 intimate connection with the lower part of the tertiary, 

 and has yielded the extraordinary horned dinosaurs, 

 pre^"iously noticed in Knowledge in the article on " Giant 

 Land Reptiles." From these Laramie cretaceous rocks 

 Professor Marsh has succeeded in obtaining a quantity of 

 teeth of mammals, although these are, unfortunately, 

 mostly found detached. These teeth indicate mammals 

 closely allied to Plivjiaulii.c of the Jurassic, and also others 

 of a carnivorous type related to the Amphithere , or some of 

 the many-molared carnivorous forms from the Dorsetshire 

 Pirrbeck. Mammals of the Triconodont type — that is, 

 those with the three cusps of the molars in a straight line 

 — seem, however, by this time to have totally disappeared. 



At a still later date a single tooth of the Plwiiaulax type 

 has been obtained from the English wealden, indicating 

 that at that epoch the Purbeek mammals still survived 

 in Europe, and leading to the hope that future researches 

 will yield us further evidence of the European mammahan 

 faima of the wealden. 



The present state of our knowledge, therefore, shows 

 that from nearly the lowest beds of the secondary period 

 tiU the close of that vast epoch, there existed a numerous 

 fauna of small mammals distributed over a large portion 

 of the globe, and displaying a remarkable persistence of 

 nearly similar type. It is further evident that such of 

 these mammals as exhibit a carnivorous type of dentition 

 appear to be allied to the more primitive of the existing 

 marsupials, although some of them may be more nearly 

 related to the almost equally low insectivores. On the other 

 hand, those which exhibit what appears to be an herbivorous 

 modification of dental structure, if they are related to any 

 living forms, appear to have an affinity with the modern egg- 

 laying mammals of Australia. Now the latter, together with 

 the marsupials and insectivores, being the lowest repre- 

 sentatives of mammalian life at present existing, are pre- 

 cisely such mammals as we should naturally have expected 

 to have been foreshadowed by more or less nearly allied 

 forms in the secondary rocks ; and, therefore, in this 

 respect, theoretical palaeontology is, so far as our present 

 knowledge goes, precisely in accord with actual facts. 



That the few triassic mammals at present known were 

 the earliest representatives of the class cannot, however, 

 be admitted for a moment, and we must accordingly look 

 either to the lower triassic rocks, or to those of the 

 underlying permiau (forming the top of the paliijozoic 

 series), for the discovery of such primitive types. Should 

 such ever be discovered, it is to be confidently expected 

 that they w^ih exhibit such a combination of characters 

 common to mammals, and certain extinct reptiles and 

 amphibians, that it will he very hard to say under what 

 class they will have to be ranked. 



Seeing that throughout the whole of the secondary period, 

 with the possible exception of a few lowly insectivores, 

 there is no evidence of the existence of any mammals 

 belonging to the higher placental type (under which are 

 included all linug representatives of the class save the 

 marsupial and egg-laying groups), the reader will naturally 

 enquire when such higher forms first made their appear- 

 ance. W'e answer, with the first dawn of the tertiary 

 period ; for in the very lowest eocene strata both of France 

 and the United States there are found, side by side with 

 small mammals allied to Plwjiauhtx and the marsupials 

 of the Jurassic and cretaceous, others, which, though still 

 of small size, were endently placentals. And it is very 



