1^4 
the right valve which was shown, from which, alone, the real nature of 
the hinge could not be determined. This information was, however, 
obtained by the discovery, by M. Peron, of a complete recent shell of 
this genus, on the coast of New Holland, on Capt. King’s Island. Some 
separate valves of the same shell, a little injured by the air and by fric- 
tion, have also been picked up at Maria’s Island, and at the island of 
Kangaroos.* 
Trigonia Margaritacea is the name given by Lamarck to this recent 
shell. It is somewhat of an orbicular form ; appearing, when the valves 
are closed, not unlike a cardium, with hardly any cartilaginal area. 
Twenty -two longitudinal ribs, crenulated through their whole length by 
crescent-formed and granular tubercles, diverge from the hinge to the 
margin. The internal surface of the shell is covered with a brilliant, 
silverish, and slightly iridiscent nacre. The muscular impressions are 
two, and are placed laterally, near to the teeth of the hinge. 
In this specimen, the real nature of the hinge was ascertained. On 
the right valve are two flat teeth, which are rather thick, and transversely 
grooved on each side ; and on the left valve are four, or rather two, 
double teeth, which, as well as those in the left valve, diverge from each 
other in their upper part. These teeth are only grooved on their inner 
side, in the cavity formed by each double tooth : this cavity, and the 
small transverse grooves, being fitted to receive the teeth of the oppo- 
site valve, and to correspond with their alternate ridges and grooves. 
A view of the hinge of this shell is given Plate XII. Fig. 1, showing the 
structure of the part of the hinge belonging to the right valve ; and 
Fig. 2, the corresponding part of the hinge belonging to the left valve. 
At about the time when this discovery was made, Dr. Menish, who 
had obtained some of these shells, as well as some masses of the Black- 
down sandstone, containing shells, from his friend Mr. Clarke, of Brid- 
well, was so fortunate as to remove the matrix from the inside of the 
* Ann. du Mus. Tome IV. p. 351. 
