Living and Extinct Mammalia. 419 



twenty-four figures in the text are brought within the comprehension 

 of the youngest student of comparative anatomy. 



Chapters iii. and iv. (pp. 82 to 116) are devoted to the Origin and 

 Classification, and to the geographical and geological distribution 

 of the class, and will be read with deep interest by all who seek to 

 trace the beginnings of Mammalian life on our earth. 



So long ago as 1879 Prof. Huxley came to the conclusion, that in 

 looking among the Vertebrates for the progenitors of the Mammalia 

 we must pass over all known forms of Birds and Reptiles and go 

 straight down to the Amphibia, "they are the only air-breathing 

 Vertebrates which, like Mammals, have a dicondylian skull. It is 

 only in them that the articular element of the mandibular arch 

 remains cartilaginous, while the quadrate ossification is small, and 

 the squamosal extends down over it to the osseous elements of the 

 mandible, thus affording an easy transition to the mammalian con- 

 dition of those parts. The pectoral girdle of the Monotremes is as 

 much amphibian as it is sauropsidian ; the carpus and the tarsus of 

 all Sauropsida, except the Chelonia, are modified away from the 

 Urodele type, while those of the Mammal are directly reducible 

 to it," In 1885 Prof. Cope called attention to the remarkable re- 

 semblance to the Monotremes presented by the skeleton of that 

 group of early Secondary reptiles which he then designated the 

 Theromorpha, but which may be included in the Anomodontia of Sir 

 Richard Owen, and came to the conclusion that in that group we 

 have the true ancestors of the Mammalia. 



" Since that date observations made on the structure of the South 

 African Anomodontia have shown such an intimate connexion between 

 this group and the Labyrinthodont Amphibians, that there can be no 

 hesitation in regarding the one as the direct descendant of the other ; 

 and we may probably regard the Mammalia as having originated 

 from the same ancestral stock at the time the Amphibian type was 

 passing into the Reptilian. From this point of view, some of the 

 mammalian features found in the more specialized Anomodonts may 

 probably be regarded as having been acquired during a parallel line 

 of development." 



In dealing with existing forms of Mammals we find that they have 

 become so broken up into distinct groups by the extinction of inter- 

 mediate forms, that a systematic classification is perfectly practicable. 



When, however, we pass to the extinct world, all is changed. In 

 many cases the boundaries of our groups become enlarged until they 

 touch those of others. New forms are discovered which cannot be 

 placed within any of the existing divisions, and they are no longer 

 sufficient for the purpose, and some other method will have to be 

 invented to show the complex relationships existing between dif- 

 ferent animal forms when viewed as a whole. 



We quite agree with the authors that a linear classification cannot 

 be made to express the many inter-relationships existing between 

 the different families and orders ; indeed, as regards many of the 

 fossil forms, it is almost impossible to decide where to place them 

 in relation to living families. 



