734 CLASS xn. 



&c. Many extinct fossil species also of this genus occur, whilst the newer 

 tertiary formations contain also the named living species in a fossil state. 

 See on these fossil species LAMARCK Ann. du Mm. vi. pp. 215 217, ix. 

 PL 1 8, figs. 6 9, and DESHAYES Descr. des Coquilles foss. des environs de 

 Paris. 



Family VII. Trigoniacea. Mantle cloven anteriorly, trachea 

 none. Foot not byssiferous, serving for creeping. Shell equivalve, 

 inequilateral, hinge with several teeth, arranged in two rows 

 diverging from the point. Points contiguous, or slightly remote. 

 Two muscular impressions remote ; internal surface of valves often 

 pearly. 



Nucula LAM. Shell transverse, with points contiguous. Hinge 

 linear, broken, interrupted by a pit for receiving the internal liga- 

 ment, with numerous teeth. 



Sp. Nucula margaritacea LAM., Area nucleus L., BLAINV. Malac. PI. 65, 

 fig. 5, DESHAYES Conchyl. PL 34, figs, u 13; in the North Sea and Medi- 

 terranean. This species also occurs fossil. The foot of these conchifers 

 resembles that of Peclunculus. This genus, which has filiform gills, might 

 be left in the preceding family. Of late years many species have become 

 known, especially through the English voyager GUMMING. They are all 

 small conchifers. 



Trigonia BRUG. Shell trigonal, sometimes suborbicular. Car- 

 dinal teeth oblong, grooved. Ligament external, marginal. Foot 

 elongate, geniculate, with the apical portion received in a groove 

 on the inferior surface of the basal portion. 



Sp. Trigonia pectinata LAM., Trigon. margaritacea, Ann. du Mus. iv. pp. 

 355 357> PI- 67, fig. i, BLAINVILLE Malac. PL 70, fig. i; in the S. 

 Pacific. This is the only species now living which is known of this genus, 

 of which numerous fossil species occur, partly in the older secondary 

 formations. Here belongs also the genus Myophoria BRONN Leth. geogn. 

 I. pp. 168 174, from the Muschel-TcalTc. Especially, however, are the 

 species of Trigonia numerous in the Jura- and Chalk-formations. Comp. 

 also AGASSIZ Etudes critiques sur les Mollusques, ire livraison, Neufchatel, 

 1840, 4to. 



Family VIII. Namdea. Mantle mostly cloven, sometimes 

 concrete posteriorly and produced into two short tracheae. Foot 

 compressed, byssus none. Four oval buccal tentacles. Shell 

 transverse, inequilateral, pearly within. Two muscular impressions 

 distinct, remote. Ligament external. 



This family contains fresh-water conchifers. LINNAEUS united the 

 species known to him partly with Mya, partly with Mytilus. They 

 might perhaps be all united in one large genus Unio. 



