MOLLUSCA: DISTRIBUTION. 313 



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 HippuritidcE is exclusively confined to the 

 Secondary rocks, and is, indeed, not known to occur beyond 

 the limits of the Cretaceous formation. The Veneridce, which 

 are perhaps the most highly organised of the families of the 

 Lameliibranchiata, 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 Secon- 

 dary 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 Heteropoda are likewise of very ancient origin, having 

 commenced their existence in the lowest Silurian deposits. 

 The genera Bellerophon, Porcellia, Cyrtolites, and Maclurea, 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 Limnaa, Paludina, 

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

 represented. These, however, are entirely Mesozoic and 

 Kainozoic. In the Palaeozoic period the sole known represen- 

 tatives of the Pulmonifera are the Pupa vetusta and Zonites 

 priscus of the Carboniferous 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 

 Pteropods, are of comparatively gigantic size, and extend from 

 the Lower Silurian to the Carboniferous period. The Silurian 

 fossil, Tentaculites, is asserted by M. Barrande to be a Pteropod, 

 but it is usually looked upon as a tubicolous Annelide. The 

 recent genus Hyalea is represented in the Tertiary period 

 (Miocene). 



