GEOGNOSY AND GEOLOGY. 171 
each other, and occupying respectively the centres of the vertex and base. 
The shell is furnished with large warts, on which are set large club-shaped 
spines, which, however, are generally broken off and dispersed. We find 
Cidaris (Hemicidaris) crenularis (pl. 38, figs. 21¢ and 21°) in the coral-rag, 
together with species of Cidaris blumenbachii (fig. 22). The zones on 
which the ambulacra stand generally run to a point. This, however, is not 
the case in Dysaster; here the rays unite in two points at no great 
distance from each other. The mouth of Dysaster lies anteriorly and below, 
while the anus is situated in the posterior border: D. capistratus ( fig. 23). 
from the Oxford clay. The genus Clypeus is characterized by a rounded 
form, a mouth on the inferior surface, a lateral anus situated in a furrow 
above the mouth, and by the convergence of the ambulacral aree to a point 
in the vertex. Clypeus hugi ( fig. 24) is found in the restricted Jura group. 
The genus Diadema, of which D. seriale (fig. 25) is found in the lias, is 
also of spheroidal form, and has ambulacral arez disposed in pairs, which 
run very regularly to the upper part. Two series of tubercles stand 
between the ambulacral aree, all appearing to have borne spines. 
A few species of the genus Spirifer recur in the lias only to disappear 
from the fossiliferous series; S. walcotii (fig. 26) is met with most 
frequently in gryphital limestone. In the Jurassic rocks, Terebratule, in 
great number, appear to replace the Spirifers; thus Terebratula numismalis 
( fig. 27) occurs in the lias, 7. globata in the inferior Jura, and T. impressa 
( fig. 28), as also T. thurmanni, in the Oxford clay. 
Of Monomyaria, Ostrea (oysters) here make their first appearance. 
They form entire banks, as at the present time. The genus Ostrea has a 
toothless hinge, through which passes a cavity containing a ligament, 
with deep muscular impressions. Ostrea marshii (fig. 29) occurs 
abundantly in the Oxford clay, and O. deltoidea ( fig. 30) in the Kimmeridge 
beds. 
The Gryphee are closely allied to the oysters, and are distinguished by 
the greater curvature and spiral twist of the beak of the larger shell. Two 
species are found in the lias, Gryphea arcuata ( fig. 31) and G. cymbium. 
G. dilatata (fig. 32) is met with in the Oxford clay, and G. virgula ( fig. 33) 
in the Kimmeridge marl. 
Plicatula is another genus allied to Ostrea. The shell is regularly 
plicated, and the beaks are not projecting; they are found in the lias: P. 
spinosa (fig. 34). 
Of the Pectens, Pecten lens (fig. 35) is found in the lower oolite, P. 
disciformis ( fig. 36) in the middle oolite, and P. personatus ( fig. 37) in the 
has. 
The genus Perna differs essentially from the preceding in shape, in the 
long hinge, and in the extensive emargination in the anterior portion of the 
shell: Perna mytiloides (pl. 38, fig. 38). 
The thick-shelled Trigonie have considerable resemblance to the 
Myophorie already referred to under the muschelkalk. They are more or 
less triangular, and have long, laterally compressed and furrowed cardinal 
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