MOLLUSCA: DISTRIBUTION, 337 



The genera Bellerophony P^rcellia, 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 Limncea, Physa, 

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

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

 Paleozoic period the sole known representatives of the PtU- 

 monifera are the Pupa tietusta 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 

 Pteropods, are of comparatively gigantic size, and extend from 

 the Lower Silurian to the Carboniferous period. The Silurian 

 fossil, TentacuUtes, is asserted by M. Barrande to be a Ptero- 

 pod, but it is usually looked upon as a tubicolous Annelide. 

 The recent genus Hyalea is represented in the Tertiary period 

 (Miocene). 



Cephalopoda. — The Cephalopods are largely represented in 

 all the primary groups of stratified rocks from the Lower Silu- 

 rian up to the present day. Of the two Orders of Cephalopoda, 

 the Tetrabranchiata is the oldest, attaining its maximum in the 

 Palaeozoic period, decreasing in the Mesozoic and Kainozoic 

 epochs, and being represented at the present day by the single 

 form Nautilus pomptlius. Of the sections of this order, the 

 Nautilidce proper and the Orthoceratidm are pre-eminently 

 Palaeozoic, and the Ammonitida are not only pre-eminently 

 but are almost exclusively Secondary. Of the abundance of 

 the two former families in the Silurian seas some idea may be 

 obtained when it is mentioned that over a thousand species 

 have been described by M, Barrande from the Silurian basin 

 of Bohemia alone. The NautilidcR proper have gradually 

 decreased in numbers from the Palaeozoic through the Second- 

 ary and Tertiary periods to the present day. The Orthoceratidce 

 died out much sooner, being exclusively Pateozoic, with the 

 exception of the genera Orthoceras itself a.nd Cyrtoceras, which 

 survived into the commencement of the Secondary period, 

 finally dying out in the Trias. 



The second family of the Tetrabranchiata — viz., the Ammo- 

 nitidcB — is almost exclusively Secondary, being very largely 

 represented by numerous species of the genera Ammonites, 



