ST. HILDA'S SNAKE-STONES. 103 



view from one of the windows at the back of my 

 house. Here the Middle Lias may be examined ; 

 and the lovely A. margaritatus, of pearly lustre, 

 along with A. capricornis, of ruder aspect, supply the 

 standards of the zones. The deeper portions of 

 the Lias may nearly all be seen by the enthusiastic 

 collector who will go out to two or three of the 

 villages south of Lincoln, such as Waddington and 

 Leadenham. It is certainly very remarkable that 

 the respective Ammonites of these closely related 



FlG. 17. A. communis. FIG. 18. Side view. 



zones should present such differences. It is one 

 of those curious and profound problems which the 

 geologist is continually meeting with. For those 

 who may wish a more detailed and systematic 

 account of these zones, I may mention that a com- 

 plete list of them, formed by a comparison of the 

 Liassic beds of the whole country, is given on p. 786 of 

 Geikie's larger Geology. Mr. Etheridge has shown 

 that these zones hold good for the whole of the 

 Secondaries throughout Asia, Europe, and America. 



