Reports and Proceedings — Royal Microscopical Society. 379 



of their shells. Chama, which represents 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 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 Bentalium 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, Eyolifhes, Conularia, are very 

 doubtfully related to the Pteropoda. We have Tentaculites in 

 Silurian and Devonian rocks ; HyolitJies, Cambrian to Permian ; and 

 Conularia, from Ordovician to Lias : both the latter are very 

 persistent types. 



In the Mollusca— Gasteropoda— PaieZZrt-like forms have existed 

 from early Palceozoic times. Walcott has figured 6 species of 

 Scenella, 8 species or varieties of Stenotheca, and 1 of Flatyceras, 

 from the Lower Cambrian of North America. Capulus has persisted 

 from Cambrian times to the present day. 



The remarkable genus Fleurotomaria also ranges from Cambrian 

 to recent, living in Japan and in the West Indies, and is represented 

 by 4 or 5 species recent, 11 Tertiary, 575 Secondary, and 570 

 Palseozoic forms. 



The Nerineidee have very specialised shells in the structure of the 

 columella; their range is also very brief. There are 150 species 

 recorded from Mesozoic strata. 



The Pulmonifera, Land-Snails, range from the Coal-measures to 

 recent. 



Among the Cephalopoda, the Nautiloid type is remarkable for its 

 persistence since Cambrian times. Many specialised forms, showing 

 extreme variety of growth and shell-structure, have branched out 

 from this stock during its dominance in Palgeozoic times, but these 

 have in turn all died out. Of these, the simple genus Ortlioceras, 

 with its long straig-ht shell, had the greatest range, viz. from the 

 Cambrian to the Trias ; other forms of Cephalopod shell have fairly 

 long ranges and show remarkable varieties of shell-structure. 



The Ammonites, which range from the Trias to the Chalk, exhibit 

 almost endless variety in shell-ornament within certain limits, and 

 have a world-wide range in Jurassic times, branching out into more 

 than 600 species. In the Cretaceous period (before their dis- 

 appearance) they put on most singular and remarkable developments 

 of shell-variation, Crioceras, Scaphites, Ancycloceras, Helicoceras, 

 Toxoceras, Baculites, Ptychoceras, Hamites, Turrilites ; then they 

 disappear entirely. We do not know the animal in Ammonites. 



