xi THE GEOLOGICAL RECORD 213 



teeth were found in what is termed the dirt-bed at 

 Swanage, in the upper part of the Jurassic formation. Two 

 of these Spalacotherium and Triconodon are here repre- 

 sented, and show how well they are preserved (Figs. 68 and 

 69). 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 Wyoming, 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 Multituberculata, from 

 the curiously tubercled teeth, resembling those of the 

 Australian ornithorhynchus. Somewhat similar teeth and 

 jaws have been found also in the Upper Cretaceous beds of 

 North America. 



Now it is quite certain that these small mammals, so 

 widely spread over the northern hemisphere, must have been 

 developed through a series of earlier forms, thus extending 

 back into that unknown gap between the Palaeozoic and 

 Mesozoic eras, and being throughout contemporaneous with 

 the great Age of Reptiles we have just been considering. 

 Yet during the whole of this vast period they apparently 

 never increased beyond 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 development for so long a period is 

 altogether unexampled in the geological record. 



The Earliest Birds 



Birds present us with a similar problem, but in their 

 case it is less extraordinary because their preservation is so 

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

 we know they must have been abundant The very earliest- 

 known fossil bird is from the Upper Jurassic of Bavaria, and 

 is beautifully preserved in the fine-grained beds of litho- 

 graphic stone. The illustration on next page is from an 

 exact drawing of this specimen (Fig. 70), in order to render 



