The Lamellibranchs are known to have existed in the Upper Cambrian period, and have steadily increased up to the present day, when the class appears 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. Upon the whole, the Asiphonate bivalves are more characteristically Palaeozoic, whilst those in which the mantle-lobes are united and there are respiratory siphons, are chiefly found in the Secondary and Tertiary epochs. One very singular and aberrant family - viz., the Hippuritidae - is exclusively confined to the Secondary rocks, and is, indeed, not known to occur beyond the limits of the Cretaceous formation. The Veneridae, which are perhaps the most highly organised of the families of the Lamellibranchiata, appear for the first time in the Oolitic rocks, and increasing in the Tertiary period, have culminated in the recent period.