734 CLASS XII. 
&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 LAmMARCK Ann. du Mus. VI. pp. 215—217, IX. 
Pl. 18, figs. 6—9, and DesHayeEs 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. 
T'wo 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., Arca nucleus L., Buatiny. Malac. Pl. 65, 
fig. 5, DesHayEs Conchyl. Pl. 34, figs. 11—13; in the North Sea and Medi- 
terranean. This species also occurs fossil. The foot of these conchifers 
resembles that of Pectunculus. 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 Cummine. They are all 
small conchifers. 
Trigonia Bruce. 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. TZrigonia pectinata Lam., Trigon. margaritacea, Ann. du Mus. iv. pp. 
355—357, Pl. 67, fig. 1, BuainvintE Malac. Pl. 70, fig. 1; 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-kalk. Especially, however, are the 
species of Zrigonia numerous in the Jura- and Chalk-formations. Comp, 
also AGASSIZ Ltudes critiques sur les Mollusques, ire livraison, Neufchatel, 
1840, 4to. 
Family VIII. Natadea. Mantle mostly cloven, sometimes - 
concrete posteriorly and produced into two short trachee. oot 
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. Liynaus united the 
species known to him partly with Mya, partly with Mytilus. They 
might perhaps be all united in one large genus Unio. 
