174 



the right valve which was shown, from which, alone, the real nature of 

 the hinge could not be determined. This information was however ob- 

 tained 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 

 Kanguroo's*. 



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 cardiurn, 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, sil- 

 verish, 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 trans- 

 versely 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 op- 

 posite 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. 35 I. 



