412 ‘ THE MEDALS OF CREATION. Cuap. XI. 
° 
Tricontag Lign. 127, fig. 1, 2, 4.—These bivalves are 
related to the Arcadece and Nucule, but distinguished by 
the peculiar character of the hinge; the right valve has 
two large oblong teeth, which diverge from the umbo, and 
are strongly furrowed, and fit into two corresponding grooved 
cavities, in the opposite, or left valve. These shells are very 
thick and nacreous; they abound in certain strata of the 
Oolite and lower Cretaceous formation, but have not been ob- 
served in any deposits of this country older than the Lias; 
there are nearly thirty British species. Two living species 
of Trigonia (Zrigonia margaritacea and 7’. Jukesii) are 
known, both inhabitants of the seas of New Holland, where 
they are associated with Terebratule. Some of the argilla- 
ceous beds of the Oolite, as the Oxford and Kimmeridge 
clays, abound in Trigoniz ; Osmington and Radipole, near 
Weymouth, are celebrated localities for these fossil shells, 
which are found there in great perfection; and on the 
French coast, where similar strata appear, the Trigoniz are 
equally abundant. Under the cliffs, near Boulogne harbour, 
the shore is strewn with them. Three common species are 
figured in Lign. 127. The casts of most of the species are 
smooth, as in fig. 2; and the collector should, therefore, 
search for impressions of the outer surface, when the shell 
is absent, as is generally the case in the Portland Oolite and 
Shanklin Sand, in which Trigoniz are very numerous. 
Near Highworth, in Wiltshire, very fine and large examples 
of Lrigonia costata, fig. 4, occur, with the shell preserved. 
The impressions of the large, oblong, diverging teeth of the 
hinge, are usually so strongly marked in the casts, as to 
render it easy to identify the shells of this genus. The 
quarries of the Portland Oolite at Swindon, Wilts, teem 
with casts of Trigonie, collocated with Ammonites. In the 
Isle of Portland they are also very numerous, some beds of 
stone being so friable, from the numerous cavities left by 
the removal of the substance of the shells, as to be unfit 
