144 INVERTEBRATE PALAEONTOLOGY 



these excellent qualities, which result in abundance of 

 palaeontological material, Mollusca are, in the main, 

 unsatisfactory fossils. The shells are wholly external 

 structures, and so usually fail to receive much impres- 

 sion from perishable morphological characters. Study 

 of recent types shows that the features of chief import- 

 ance in taxonomy are sensory, respiratory, reproductive 

 and masticatory organs. All evidence of these qualities 

 is usually inaccessible in fossil material, so that the 

 classification of extinct Molluscs is inevitably somewhat 

 empirical. Stern necessity, and fortunate coincidence, 

 have made the Cephalopoda reveal characters of taxo- 

 nomic value; it is probable that further researches on 

 Pelecypods and Gastropods may make palaeontological 

 classification possible. Moderately successful attempts 

 have been made in the case of Bivalves, but Univalves 

 are still grouped into families by analogy rather than 

 morphology. 



Of the five classes of Molluscs suitable for fossiliza- 

 tion, Gastropods alone are definitely recognized in the 

 Cambrian. It is possible that Pelecypods and Cepha- 

 lopods may occur, but much uncertainty surrounds 

 the obscure and primitive types recorded. Cambrian 

 " Bivalves " have on several occasions proved to be 

 Branchiopod Crustacea (or even distorted Brachiopods) ; 

 while presumed Orthoceras- forms from that horizon 

 might be ascribed to utterly different organisms with 

 equal probability. Both Pelecypods and Cephalopods 

 were represented in the Ordovician, where they were 

 accompanied by the earliest Amphineura. Scaphopods 

 possibly occur in the Silurian, but their simple tubular 

 shells are difficult to distinguish from those of Pteropods 

 or even Tubicolous Annelids. 



The Pelecypoda are divided into three subclasses, 

 Prionodesmacea, Anomalodesmacea and Teleodesmacea. 



