1 01 



TRIGONIA. 



TESTA aequivalvis, insequilatera, transversa, 

 trigona, interdum suborbicularis. Denies car- 

 clinales oblongi, lateraliter compressi, divari- 

 eati ; duo in valva altera, utroque latere trans- 

 versim sulcati, quatuor in altera, uno tantum 

 latere sulcati. Ligamentum externum, crassiim, 

 marginale. Impressiones musculares d\\8B, i 



One of the most strongly marked genera of regular 

 l)iYalves established by Bruguiere^ and adopted by all 

 succeeding writers ; indeed, we do not conceive bow it 

 can be placed in any Linnean Genus; its extraordinary 

 characters, which we shall now proceed to detail, will 

 show how it is to be distinguished from all other genera. 



Shell equivalve, mostly inequilateral, transverse, 

 rather triangular, whence its generic name; sometimes, 

 but very seldom suborbicular. Cardinal teeth oblong, 

 laterally compressed, divaricated, two in one valve, 

 transversely grooved on both sides, the grooves regular- 

 ly marked, and each forming a segment of a circle; 

 four in the other, which are grooved on one side onl]^ 

 but alternately in pairs, so that the four teeth of this 

 valve receive within their grooved sides the two teeth of 

 the other valve. Ligament external, thick, rather short, 

 marginal. Principal muscular impressions two, lateral, 

 very distinct, one of them placed close to the upper end 

 of the hinge tooth, and rather behind it ; the other rather 

 more distant, with another very minute one between it 

 and the hinge tooth. Muscular impression of the mantle 

 very nearly entire. 



Only one recent species of this Genus is known ; it 

 is a shell of extreme scarcity, which has been brought 



