MOLLUSCA : DISTRIBUTION. 359 



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 chiefly found in the 

 Secondary and Tertiary epochs. One very singular and aber- 

 rant family viz., the Hippuritida is exclusively confined to 

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

 yond the limits of the Cretaceous formation. The Venerida, 

 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 Holostomata 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. 

 The genera Bellerophon, Porcellia, Cyrtolites, and Madurea, 

 are almost exclusively Palaeozoic ; Bellerophina is found in the 

 Gault (Secondary), and Carinaria has been detected in the 

 Tertiaries. 



The Pulmonate Gasteropoda, as was to be anticipated, are 

 not found abundantly as fossils, occurring chiefly in lacustrine 

 and estuarine deposits, in which the genera Limncea, Physa, 

 Ancylus, &c., are amongst those most commonly represented. 

 These, however, are entirely Mesozoic and Kainozoic. In the 

 Palaeozoic period the sole known representatives of the Pul- 

 monifera are the Pupa vetusta and Zonites priscus of the Car- 

 boniferous rocks. 



Pteropoda. The Pteropods are not largely represented in 

 fossiliferous deposits, but they have a wide range in time, ex- 

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

 The Theca and Conularia of the Palaeozoic period, if truly 



