On the Mesozoic Mammalia. 21 



work now before us, these are brought up to no less than thirty- 

 five, five of which come irom the Trias. A very convenient 

 table shows theorder of occurrence of these forms in time, so far 

 as they are now known to us, the arrangement in the columns 

 presenting the beds of England, North America, Africa, Austra- 

 lia, and the Continent. It is pointed out by our author, that, 

 "In a rapid survey of this ancient fauna, we are at first struck with 

 the very great diversity which prevails in the form and arrange- 

 ment of the teeth, consisting of six or seven wholly distinct types; 

 and this at a zoological period which we have been accustomed 

 to consider as the dawn of mammalian life. The above types, 

 although primitive, are essentially mammalian. In one genus 

 only, Dromatherium, do we find clear evidence of reptilian affin- 

 ity in the dentition. Then we are surprised to discover a very 

 close zoological relationship between fossil faunae of the same 

 age, but having a wide geographical distribution. The most 

 striking instance of this is the parallelism between the American 

 and British upper Jurassic fauna." These , mesozoic mammals 

 were characterized by their diminutive size, and the excessive 

 number of teeth they possessed, which latter, otherwise, are less 

 archaic in their pattern than we would be led to believe from a 

 mere casual observation; the molars being in unusual numbers. 

 As his task developed, Professor Osborn was led to lay especial 

 stress upon (1) the generic characters of the British mesozoic 

 forms; to show (2) their relationships to modern orders, that is 

 all the known mesozoic species to the existing orders of mammals; 

 and (3), a full discussion of the dentition in all its bearings. This 

 is the fundamental plan of the monograph before us, and upon 

 these lines it has been worked out with marked ability and clear- 

 ness. The literature of the subject has been carefully gone over, 

 and previous laborers in the same fields given in every instance, 

 full credit for their productions. Space will not admit of our 

 entering in detail upon the admirable classification adopted in 

 this monograph, be it enough to say that it is based almost ex- 

 clusively on the dentition, and that, "We first observe that the 

 Mesozoic Mammalia divide into two large groups. In the first 

 group, A, one of the incisors is greatly developed at the expense 

 of the others, and of the canine, which usually disappears; be- 

 hind these teeth is a diastema of varying width, while the molars 

 bear numerous tubercles. In the second group, B, the incisors 

 are small and numerous, the canine is always present, and well 

 developed; the teeth usually form a continuous series, and the 

 molars bear cusps instead of tubercles. These two divisions sug- 

 gest those which obtain among the modern Marsupials, but are 

 in fact much more sharply defined and widely separated from 

 each other." 



In conclusion, we find a general discussion upon the develop- 

 ment of the teeth of these mammals in time, and masterly sec- 

 tions devoted to the zoological position of these mesozoic types 

 in the two groups just mentioned, to their relations to the Mar- 



