1?2 
allow of this being determined. A representation of the inside of this 
shell is given Plate XIII. Fig. 1. 
Cl. Trigonia. A trigonal or suborbicular inequilateral bivalve. On 
the right valve are two oblong, flat, diverging hinge teeth, transversely 
grooved on each side : on the left valve, four flat hinge teeth, transversely 
grooved on one side only, disposed in pairs, each pair diverging and ex- 
actly receiving those of the opposite valve. 
These shells have been long known and admired in their fossil state, 
both in this island and on the continent. The first notice that I find of 
them is that of Langius, who gives a figure of one of these shells, Hist, 
lap. Jig. Helv. Tab. 44, Fig. 5, and speaks of it as Cone kites Helve thus 
visu prodigiosus, triqueterus striatus, and thinks it resembles Concha 
indica visu prodigiosa, Bonanni, No. 91. 
Our English naturalists very early noticed this curious fossil. Dr. Plot 
gives to a cast of this genus the name of Hippocephaloides, Hist, of 
Oxfordshire , Plate vn. Fig. 1 . ; and Lhwydd, who gives to these shells 
the name of Curvirostrce , mentions several ; such as, C. rugosa clavel- 
lata major , from Garsington ; rugosa minor , clavis aut nullis, aut pau- 
cissimis cegre conspicuis donata, from Buckinghamshire ; rugosa major 
non clavata, seu levibus rugis ex sculp ta, from Bullington, Lithol. No. 
700, &c. From these notices of Lhwydd it appears, that, at least, three 
species of this genus were known to him to have existed in this island. 
Da Costa speaks only of these shells, to express his doubts, whether to 
place them under Cunei or Areas, no knowledge having then been ob- 
tained as to the kind of hinge which they possessed. 
M. Walch, who was very much at a loss in what genus to dispose of 
them, was disposed to place them among the Veneres impuberes of Lin- 
naeus. Of these shells, he saw three species : complete shells of T. cla- 
vellata and T. costata, and a fragment of T. aliformis ; but, being unac- 
quainted with the hinge, he does not appear to have been fully aware 
of their relationship. Speaking of the latter of these fossils, he makes 
the following remarks on the curious circumstance of the characters of 
