MIDDLE LIAS FOSSILS. 
191 
It must be remembered that, owing to the prevalence of Am. 
margaritatus in the upper beds, the occurrence of this index- 
species is not to be depended upon for the identification of the 
zone. A. spinatus also (though rarely) has been recorded from 
the lower beds, so that the distinction between these zones is for 
the most part a matter of local convenience, dependent mainly 
on the development of the Rock Bed in the upper part of the 
formation. Personally I am unable to confirm the statement that 
A. spinatus occurs in the lower beds of the Middle Lias. (See 
p. 240). 
The more abundant and characteristic fossils of the Middle 
Lias may be stated as follows : 
Zone of Ammonites margaritatus. (Fig. 54.) 
Belemnites clavatus. (Fig. 22, p. 47.) 
elongatus. 
Area Stricklandi. 
Avicula (Monotis) insequivalvis (sinemuriensis). 
Cardinal (Protocardium) truncatum. (Fig. 58.) 
Cypricardia intermedia. 
Gresslya intermedia. 
lunulata. 
Leda complauata. 
Modiola scalprum. (Fig. 56.) 
Pecten lunularis (liasinus). 
Pholadomya ambigua. 
Pleuromya costata. 
Plicatula spinosa. (Fig. 31, p. 50.) 
Ditrupa circinata. 
Ophioderma Egertoni. (Fig. 64.) 
Milleri. (Fig. 65.) 
MIDDLE LIAS FOSSILS. 
FIG. 61. 
FIG. 61. Rhynchonella tetrahedra, Sow. (nat. size). 
