LAMELLIBR ANCHI ATA. 2 1 9 



and is secreted by a special gland. Others are fixed to some 

 solid body by the substance of one of the valves. Many, such 

 as the Myas, spend their existence sunk in the sand of the sea- 

 shore or in the mud of estuaries. Others, as 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 pre- 

 sent 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 de- 

 posits the Bivalves belong mainly to the group of the Asiph- 

 onida, in which there are no respiratory siphons. In the later 

 Secondary and Tertiary Rocks, on the other hand, there is a 

 predominance of Siphonate Bivalves, in which the mantle-lobes 

 are united and there are respiratory siphons. Upon the whole, 

 the Lamellibranchiata are sparingly represented 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, profusely represented in the 

 Jurassic Rocks, and very abundant in the Cretaceous and Ter- 

 tiary periods (Lobley). In the Carboniferous Rocks the family 

 of the Aviculidce is especially abundant. One very singular 

 and aberrant family viz., the Hippuritida is exclusively con- 

 fined to the Secondary period, and is not known to occur out 

 of the limits of the Cretaceous formation. The Venerida, 

 which are perhaps the most highly organised of the Lamelli- 

 branchiata, appear for the first time in the Oolitic Rocks, and, 

 increasing in the Tertiary period, have culminated in the 

 Recent period. The remains of Lamellibranchiata are very 

 abundant in many formations, and are of great palaeontological 

 importance. It will therefore be well to review the families* 

 of the class briefly, giving the leading characters, more impor- 

 tant genera, and geological distribution of each. 



SECTION A. ASIPHONIDA. 



FAM. i. OSTREID^E. Shell inequivalve, slightly inequilateral, 

 free or attached ; hinge usually edentulous. Ligament internal. 



* In the following synoptical view of the Lamellibranchiata, the classi- 

 fication adopted in Woodward's admirable Manual of the Mollusca has 

 been mainly followed. 



