150 Transactions of the Society. 



Many species construct tubes. These variously formed cases 

 (called Serpulce) are common in many formations, but do not dis- 

 close much information about the structure of the animal itself. 



They admirably illustrate the persistence in time of very simple 

 and lowly organised forms, having bodies composed of a large 

 number of similar segments (often capable of subdivision), and 

 possessing moreover great powers of reparation and reproduction 

 and resistance to death. 



VIII. MOLLUSCA. 



In the Mollusc a, amongst the Lamellie-ranchiata, there 

 are many persistent types showing very small amount of varia- 

 tion. 



E.g. Solenomya has persisted since Carboniferous times, and 

 Nucula from the Silurian onwards. Both belong to the " Proto- 

 branchiata" forms, with simple gills and a sole on the foot for 

 creeping upon — not a mere digging foot. 



In contrast to these are the Eudistes, such as Dicer as, Upper 

 Jurassic ; Bcquienia, Monopleura, Caprina, Sphcerulites, and Hip- 

 purites, &c, from the Cretaceous. These peculiar Molluscs had a 

 world-wide distribution, and occur in such numbers that beds of 

 limestone are often built up of their shells. Cham a, which re- 

 presents them, has continued to the present day, but is less 

 specialised. 



Trigonia is not only a persistent genus, but exhibits great 

 resistance to extreme variation, save in minor matters of shell- 

 ornament. It ranges from the Trias to recent, and has a world- 

 wide distribution. There are three species living in Australia and 

 at least 100 species extinct. 



In the Scaphopoda the curious tubular genus Dentalium 

 ranges from the Ordovician to the present day. There are many 

 species, but little variation from the type. 



The multivalve Chitons extend also from Ordovician times to 

 the present, but are never common in a fossil state. Only 70 

 species have been described from all known horizons. They are 

 more abundant in modern seas, more than 200 species being now 

 living. 



The Pteropods (proper) only date back to the Cretaceous. 



The earlier forms known as Tentaculites, Hyolithes, Conularia, 

 are very doubtfully related to the Pteropoda. We have Ten- 

 taculites in Silurian and Devonian rocks; Hyolithes, Cambrian to 

 Permian ; and Conularia, from Ordovician to Lias ; both the latter 

 are very persistent types. 



In the Mollusca — Gasteropoda — Patella-like forms have 

 existed from early Palaeozoic times. Walcott has figured 6 species 

 of Sccnella, 8 species or varieties of Stenotheca, and 1 of Platyccras 



