420 Reviews — Flower and Lydekker's 



On pp. 107-115 is givea a summary of what is known of the 

 Mesozoic Mammals, and on pp. 115-116, a few remarks on Tertiary 

 Mammals ; but these are dealt with in greater detail in the sub- 

 sequent chapters under the heads of the groups to which they are 

 severally allied. " The comparatively scanty evidence of mammalian 

 life hitherto yielded by the Cretaceous (formation), coupled with the 

 number and variety of forms approximating to the existing groups 

 found even in the lowest Tertiary, indicates a great imperfection of 

 the geological record. At present, indeed, we have no decisive 

 evidence of the existence of any members of the Eutherian sub- 

 class previously to the Tertiary ; but it can hardly be doubted 

 that in some part of the world they had made their appearance 

 before that epoch. The Eutherian mammals of the lowest Eocene, 

 both in Europe and the United States, are of an extremely gene- 

 ralized type ; and although many of them approximate to existing 

 groups, they show such a combination of characters, now restricted 

 to individual groups, as to indicate that several of the various orders 

 into which the subclass is now divided were at the period very 

 intimately connected. A marked feature of these early Eutherians 

 is the prevalency of trituberculism in the dentition, not less note- 

 worthy being the frequent occurrence of pentadactylism in the feet, 

 while many of the individual bones were devoid of the grooves and 

 ridges found in those of later types. By the time that we reach the 

 upper division of the Eocene period, such as the horizon of the 

 well-known gypsum of the Paris basin, nearly all the chief groups 

 of mammals had become clearly differentiated from one another, 

 although their representatives were usually more generalized than 

 their existing allies. From this date to the later geological periods 

 there is a gradual approximation to the type of mammalian life 

 existing at the present day." 



Turning to the table (pp. 88-92), we find the class Mammalia 

 subdivided into sub-classes, orders and sub-orders, and lastly into 

 families. One hundred and thirty-one families are recorded, of 

 which eighty-six are living and forty-six are extinct. Many of the 

 so-called families of extinct forms are represented by a single genus 

 founded on a fragmentary individual remain, so that, although of 

 extreme interest, they can hardly claim equal taxonomic importance 

 with known families represented by many genera all composed of 

 complete individuals. 



We notice a trivial slip (pp. 88-89), Tritylodontidai being given 

 twice over in the Prototheria and in the Metatheria. The reference 

 to it under Multituberculata is doubtless correct, and the second 

 (under Marsupialia) may be cancelled. 



Owing to the diligence of recent explorers the extinct families 

 known are now fully half as numerous as the living, nevertheless of 

 many of these our information is of the scantiest ; nor do they help 

 us to trace the derivation of, or the inter-relationship which we 

 know must have existed in the past between many of the great 

 groups which stand out to-day in strongly-marked distinctiveness 

 from the rest of the class. What, for instance, do we know of the 



