By J. C. MANSEL-PLEYDELL, Esq., President. 



| HE "genus Trigonia was placed by Brongniart among 

 the Arcada ; also by Lamarck, who subsequently 

 separated it under a new family, Trigonida, together 

 with Myophoria and Axinm. It was changed by Sowerby to 

 Lyrodon, the name being previously occupied by a genus of 

 plants. 



The Trigoniae made their first appearance in the Liassic seas, 

 and became very numerous during the deposition of the Oolitic 

 beds, especially in the upper series, when they began to show 

 symptoms of decline, so much so, that the Cretaceous formation 

 does not contain a fourth part compared with those of the 

 previous Jurassic period, At the same time they underwent a 

 material modification of form, losing their trigonal shape, becom- 

 ing more inflated and rotund, so as to give them the appearance 

 of belonging to a separate family. The living Trigoniee (whose 

 valves are channelled and bear a close affinity to the forms of 

 the Scabrate section), have only two or three representatives, 

 and which are restricted to the Australian seas. 



The special character of the genus consists in the perfect 

 symmetry of valves, which are precisely similar except in the parts 

 occupied by the hinge ; the posterior portion in some species is 

 prolonged, in others it is square or truncate; the beaks are 



