4*70 LAMELLIBRANCHIATA. 



the Pholades and Lithodomi, bore holes in rock or wood, in 

 which they live. Finally, many are permanently free and 

 locomotive. 



As regards the general distribution in time of the Lamelli- 

 branchiata, the class seems to have commenced in the Lower 

 Silurian rocks, and to have steadily increased up to the pres- 

 ent day, when it seems to have attained its maximum, both 

 as regards numbers and as regards variety of type. The 

 recent Bivalves are also superior in organisation to those 

 which have preceded them. In the Palaeozoic and earlier 

 Secondary deposits the Bivalves belong mainly to the group 

 of the Asiphonida, in which there are no respiratory siphons. 

 In the later Secondary and Tertiary rocks, on the other hand, 

 there is a predominance of Siplionate Bivalves, in which the 

 mantle-lobes are united and there are respiratory siphons. 

 Upon the whole, the Lamellibranchiata are sparingly repre- 

 sented in the Lower Silurian, more abundant in the Upper 

 Silurian, reduced in numbers in the Devonian, very plentiful 

 in the Carboniferous, scanty in the Permian and Trias, pro- 

 fusely represented in the Jurassic rocks, and very abundant 

 in the Cretaceous and Tertiary periods (Lobley). In the 

 Carboniferous rocks the family of the Aviculidce is especially 

 abundant. One very singular and aberrant family viz., the 

 Hippuritidce is exclusively confined to the Secondary period, 

 and is not known to occur out of the limits of the Cretaceous 

 formation. The Veneridce, which are perhaps the most highly 

 organised of the Lamellibranchiata, appear for the first time 

 in the Oolitic rocks, and, increasing in the Tertiary period, 

 have culminated in the Eecent period. The remains of 

 Lamellibranchiata are very abundant in many formations, 

 and are of great palseontological importance. It will there- 

 fore be well to review the families 1 of the class briefly, 

 giving the leading characters, more important genera, and 

 geological distribution of each. 



1 In the following synoptical view of the Lamellibranchiata, the classifica- 

 tion adopted in Woodward's admirable ' Manual of the Mollusca ' has been 

 mainly followed. 



