A Suggestion on Extinction. 471 



distribution, and often occur in such numbers that whole 

 beds of limestone are almost entirely built up out of their 

 shells. Their onset was very sudden, and their extinction 

 apparently as rapid. The allied form Chama, which has 

 remained since the Cretaceous, appears to owe its persistence 

 to not having taken part in this wonderful specialisation. 



Passing on to the Scaphopoda,we find the curious Dentalium, 

 ranging from the Ordovician to the present day. There are 

 many species, but the type is very strictly adhered to, and 

 the variation very slight. Its appearance is very constant, 

 but rarely in great numbers. 



The Chitons have undergone a very slowly increasing 

 specialisation from the Ordovician onwards. They are 

 never common in the fossil state, only about seventy species 

 having been described from all known horizons, and the 

 group appears to be more dominant at the present time than 

 at any former period of geological history. Their very 

 generalised position amongst the Mollusca accords well with 

 this slowly acquired specialisation and persistence of type. 



The Pteropoda are only known with certainty to date back 

 to the Cretaceous. To these the curious genera Tentaculites, 

 Hyolithes, Conularia, and their allies are very doubtfully 

 related. Tentaculites occurs in extraordinary profusion in 

 the Silurian and Devonian rocks, and then became extinct. 

 Hyolithes ranges from the Cambriftn to the Permian, and 

 Conularia from the Ordovician to the Lias, both with great 

 persistence of type. 



Amongst the Gastropoda, patella-like forms have existed 

 since the early Palaeozoic. Capulus and its allies appear 

 to have had greater dominance in early Palaeozoic times 

 than at any other period of their existence. The genus 

 Capulus itself has persisted from the Cambrian to the 

 present day. Various genera belonging to the family are 

 stationary throughout the greater part of their existence 

 attached to some foreign body, and sometimes overgrown by 

 it. It is a curious coincidence of sluggishness of habit, 

 accompanied by long duration in time. The genus Pleuro- 

 tomaria also has a range from the Cambrian to the present 

 time. It is very rare during the Tertiary, and the five living 



