404 MANUAL OF ZOOLOGY. 



the possession of long respiratory siphons, and a sinuated pallial 

 line, and it comprises the families Veneridce, Mactridce, Tellinidce, 

 Solemdce, Myaridce, Anatinidce, Gastrochtznidce, and Pholadidcz. 



The Chamidce (Thorny Clams) are fixed to foreign bodies by the sub- 

 stance of either valve indifferently, and are all inhabitants of the sea. 

 The extraordinary extinct group of the Hippuritidat, from the fossils asso- 

 ciated with them, are known to have been also marine ; and they are often 

 found in great beds like Oysters, attached to one another and to foreign 

 objects by the beak of the right valve. The Tridacnida (Giant Clams) 

 have a similar habitat, and the shell may attain a weight of five hundred 

 pounds. The Cardiadce (Cockles) and Liicinida are also marine, as are 

 the Cyprinidce ; but the Cydadidce are fresh- water and brackish- water 

 forms. The Venerida (Clams) are amongst the most beautiful of the Bi- 

 valves, and are found in all seas, attaining their maximum in warm regions. 

 The Mactridce (Trough-shells) and Tellinidcz are mostly marine, though 

 also found in brackish waters ; and the Solenidce (Razor-shells), Myacidce, 

 and Anatinida are essentially marine, though some of the Myacidce extend 

 their range for a considerable distance above the mouths of rivers. The 

 Gastrochcenidcz are all natives of the sea, and have a burrowing habit, bor- 

 ing holes for habitation in rocks, or living in the mud. Lastly, the Phola- 

 didce (Piddocks and Ship-worms) bore holes in stone or wood, in which 

 they live, and are all marine in habit. The Ship- worms (Teredo) have 

 long worm-like bodies, and do an immense amount of harm by honey- 

 combing with their burrows the sides of ships, or other wooden structures 

 immersed in the sea. 



DISTRIBUTION OF THE LAMELLIBRANCHIATA IN TIME. The 

 Lamellibranchs are known to have existed in the Upper Cam- 

 brian 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 bi- 

 valves 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 

 Hippuritidcz is exclusively confined to the Secondary rocks, 

 and is, indeed, not known to occur beyond the limits of the 

 Cretaceous formation. The Veneridcz, 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. 





