336 MANUAL OF ZOOLOGY. 



CHAPTER LII. 



DISTRIBUTION OF THE MOLLUSC A PROPER 

 IN TIME. 



Remains of the Mollusca proper are found in greater or less 

 abundance in almost all the stratified rocks from the com- 

 mencement of the Silurian period up to the present day. 

 Speaking generally, the Tetrabranchiate Cephalopoda are the 

 chief representatives of the Mollusca in the Palseozoic rocks, 

 the Lamellibranchiata and the Dibranchiate Cephalopoda in the 

 Mesozoic rocks, and the Gasteropoda in the Kainozoic period ; 

 but all the primary classes are represented even in the Lower 

 Silurian rocks. The folUowing are the more noticeable facts 

 relating to the distribution of the various classes in past time. 



Lamellibranchiata. — The Lamellibranchs are known to . have 

 existed in the Lower Silurian period, and have steadily in- 

 creased 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 cihiefly found in the 

 Secondary and Tertiary epochs. One very singular and aber- 

 rant family — viz., the Hippuritidce — is exclusively confined to 

 the Secondary rocks, and is, indeed, not known to occur be- 

 yond the limits of the Cretaceous formation. The VeneridtB, 

 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. 



Gasteropoda. — The Gasteropoda are represented in past time 

 from the Lower Silurian rocks up to the present day. Of the 

 Branchifera the Holestomata are more abundant in the Palaeo- 

 zoic period, the Siphonostomata abounding more in the Se- 

 condary and Tertiary rocks, but not attaining their maximum 

 till the present day. The place of the carnivorous Siphono- 

 stomata in the Palaeozoic seas appears to have been filled by 

 the Tetrabranchiate Cephalopods. The branchiate Gastero- 

 pods of fresh water are chiefly represented as fossils by the 

 genera Paludina, Valvata, and Ampullaria. 



The Heteropoda are likewise of very ancient origin, having 

 commenced their existence in the lowest Silurian deposits. 



