354 
ELEMENTS OF GEOLOGY. 
the Nautilus and Belemnite also abonnding. The whole 
series has been divided by zones characterized by particular 
ammonites; for while other families of shells pass from one 
division to another in numbers varying from about 20 to 50 
per cent., these cephalopods are almost always limited to 
single zones, as Quenstedt and Oppel have shown for Ger¬ 
many, and Dr. Wright and others for England. 
As no actual unconformity is known from the top of the 
Upper to the bottom of the Lower Lias, and as there is a 
marked uniformity in the mineral character of almost all the 
strata, it is somewhat difficult to account even for such partial 
breaks as have been alluded to in the succession of species, 
if we reject the hypothesis that the old species were in each 
case destroyed at the close of the deposition of the rocks con¬ 
taining them, and replaced by the creation of new forms when 
the succeeding formation began. I agree with Professor Ram¬ 
say in not accepting this hypothesis. No doubt some of the 
old species occasionally died out, and left no representatives 
in Europe or elsewhere; others were locally exterminated in 
the struggle for life by species which invaded their ancient 
domain, or by varieties better fitted for a new state of things. 
Pauses also of vast duration may have occurred in the depo¬ 
sition of strata, allowing time for the modification of organic 
life throughout the globe, slowly brought about by variation 
accompanied by extinction of the original forms. 
Fossils of the Lias.—The name of Gryphite limestone has 
sometimes been applied to the Lias, in consequence of the 
Fig. 861. 
Gryphcea incurva. Sow. {O, 
mcuata^ Lam.) Lias. 
Plagiofitoma {Lima) giganteum, Sow. 
Inferior Oolite aud Lias. 
great number of shells which it contains of a species of oyster, 
or Gryphcea (Fig. 362). A large heavy shell called Hippo- 
