172 



allow of this being determined. A representation of the inside of this 

 shell is given Plate XIII. Fig. 1. 



CI. 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. fig. Helv. Tab. 44, Fig. 5, and speaks of it as Conchites Helvetica 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 Oxford- 

 shire, Plate vu. Fig. I.; and Lhwydd, who gives to these shells the 

 name of Curvirostr<e, mentions several ; such as, C. rugosa clavellata major, 

 from Garsington ; rugosa minor, ctavis aut millis, aut paucissimis <eg?' con- 

 spicuis donata, from Buckinghamshire ; rugosa major non clavata, sen levibus 

 rugis exsculpta, from Bullington, Lithol. No. 700, &c. From these no- 

 tices 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 Area, no knowledge having then been obtained 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 



