president's addeess. 195 



Thetis, and Crassatella range from the Neocomian beds to the seas of 

 to-day ; Crenella and Chama from the Upper Greensand to Recent ; 

 Cardium from Eocene, and Tridacna from Miocene to Recent times. 



Taking next the Gastropoda, we find that the Prosobranchiata, 

 possessing as they do a calcareous shell and being of aquatic habits, 

 are largely represented in a fossil state ; though the section Hetero- 

 poda are only represented in Tertiary deposits. 



The Opisthobranchiata are only imperfectly represented in past 

 times ; the section Nudibranchiata is, of course, quite unknown in 

 a fossil state. On the other hand of the section Tectibranchiata, 

 Pteropods, belonging to the genus Hi/oUthes, are met with in con- 

 siderable numbers in the Lower Cambrian rocks, but are absent from 

 the Neozoic formations. This is a strong argument in favour of the 

 later deposits having been largely formed near shore, or in inland 

 seas, whilst the earlier deposits were laid down in wider oceanic areas. 

 Those Pulmonate Gastropoda which live habitually in fresh water 

 (as the Limneidae), are better represented in the Secondary rocks 

 than are their purely terrestrial relatives (the Helicidse), but even 

 these last are occasionally met with, and two genera have actually 

 been found as far back as in the Coal-measures of N. America. 



The Prosobranchiate Gastropoda appear in the Cambrian, from 

 which quite a number of f onus have been obtained ; of these the genera 

 Murchisonia and Fleurotomaria are amongst the most important. 



Numerous (holostomatous) Gastropods are found in the Ordovician 

 and Silurian rocks. Mcomphalus, Fleurotomaria, and Bellerophoti are 

 the most prominent types. 



The Pteropods are represented by Comdaria, Hyolithes, and 

 Tentaciilites . 



In later Palaeozoic times (Devonian, Carboniferous, and Permian) 

 the general character of the Mollusca remains unchanged, the pre- 

 dominating forms still being holostomatous Prosobranchs and 

 Pteropods. 



In the Coal-measures we meet with the earliest known forms of 

 Pulmonata, true land-dwelling, air-breathing Snails, Zonites and Fupa; 

 discovered by Sir William Dawson in the hollow, but still erect, 

 sigillaria-trees of the South Joggins Coal-field, Nova Scotia. 



In the Triassic period, the main characteristics of the Gastropods 

 are those of the earlier Palaeozoic period ; but siphonostomatous Proso- 

 branchs, such as Cerithium, Furpurina, etc., have already made their 

 appearance, and the old forms of Pteropods (save Conularia) have 

 disappeared, and modern forms, like Sti/liola, have come in. 



In the Jurassic rocks siphonostomatous Snails prevail, and the 

 first Opisthobranchs appear; whilst fresh-water genera, like Planorlis, 

 Faludina, Melaiiia, etc., are met with. 



In the Cretaceous period forms belonging to existing genera 

 continue to increase in number, and in the Tertiaries they become 

 predominant, the representatives of extinct genera being rare. 



The Patellidae (Limpets) appear as early as the Cambrian rocks 

 {e.g. Tryhlidium). The various forms of Limpet-shaped shells, with 

 a recurved apex and spiral nucleus, with the antei'ior margin notched, 



VOL. I. OCTOBER, 1894. 13a 



