~ 
714 
ae 
dant genera are Avicula, Gervillia, Gryphexa, Haogyra, Lina, 
Monotis, Ostrea, Pecten, Pinna, Astarte, Cardinia, Cardium, Gresslya, 
Hippopodium, Modiola, Myacites, Pholadomya, and Trigonia. Some of 
these genera, particularly the tribe of oysters, are specially charac- 






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Fig. 366.—Li1as CRINOIDS. 
a, Pentacrinus basaltiformis (Goldf.) (side view and end view of part of stem); 
b, Extracrinus briareus (Mill.)(). 
teristic, Gryphewa, for example, occurring in such numbers in 
some of the Lias limestones as to suggest tor these strata the 
name of “Gryphite Limestone.” 

URCHIN. 
Cidaris florigemma 
(Phill.)—Corallian. 
883-7). 
Different species of Trigonia, 
a genus now restricted to the Australian seas, 
are likewise distinctive of horizons in the 
middle and upper part of the system. Many 
of the most abundant gasteropods belong to 
still living genera, as Cerithiwm, Natica, Pur- 
pura. Dut the most important element in the 
molluscan fauna was undoubtedly supplied by 
the cephalopods. Jn particular the tetra- 
branchiate tribe of Ammonites attained an 
extraordinary exuberance, both in number of 
individuals and in variety of form (see Figs. 
The dibranchiate division was likewise represented by 
species of cuttle-fish (Teudopsis, Beloteuthis, Sepia, but particularly 
Belemnites, which is the preponderating type). 
No contrast can 
be more marked than between the crustacean fauna of the Jurassic 
; 
STRATIGRAPHICAL GEOLOGY.  [Boox VI. 
“+ 
