THE GEOLOGICAL RECORD 229 



• — small mammalian jaws with teeth — were found in what 

 is termed the dirt-bed at Swanage, in the upper part of the 

 Jurassic formation. Two of these — Spalucotherium and 

 Triconodon — are here represented, and show how well thej 

 are preserved (Figs. 68 and G9). Another of a different type 

 (Plagiaulax) has been also found in a much older formation 

 in Somersetshire — the Rhetic or Upper Trias — and in beds 

 of the same age in Bavaria, near Wiirtemberg. Both these 

 types of jaw have been since found in considerable abundance 

 in the Jurassic beds of W^'oming, U.S.A. These materials 

 have enabled palaeontologists to decide that the former group 

 were really of the marsupial type, while the latter (and 

 earlier in time) belong to a distinct sub-class, the Multituber- 

 culata, from the curiously tubercled teeth, resembling those of 

 the Australian ornithorhvnchus. Somewhat similar teeth and 

 jaws have been found also in the Upper Cretaceous beds of 

 !N"orth America. 



N^ow it is quite certain that these small mammals, so widely 

 spread over the northern hemisphere, must have been devel- 

 oped through a series of earlier forms, thus extending back 

 into that unknown gap between the Palaeozoic and Mesozoic 

 eras, and being throughout contemporaneous ^vith the great Age 

 of Reptiles w^e have just been considering. Yet during the 

 whole of this vast period they apparently never increased be- 

 yond the size of a mouse or rat, and though they diverged 

 into many varied forms, never rose above the lowly types of 

 the monotremes or the marsupials ! Such an arrest of develop- 

 ment for so long a period is altogether unexampled in the geo- 

 logical record. 



The Earliest Birds 



Birds present us with a similar problem, but in their case 

 it is less extraordinary because their ])reservation is so much 

 more rare an event, even in the Tertiary period, when we 

 know thev must have been abundant. The verv earliest-known 

 fossil bird is from the Upper Jurassic of Bavaria, and is beau- 



