154 Transactions of the Society. 



The diagram (fig. 35) on the preceding page (p. 153) is in- 

 tended to convey an idea of the probable evolution of the Arthro- 

 poda in geological time. 



Summary. 



And now, " let us hear the conclusion of the whole matter." 



The whole history, since the beginning of life on the earth, 

 shows a steady upward tendency (in fact Evolution) in life as 

 displayed in the Geological Eecord. 



Extinct Groups. 



Some forms appear, attain a more or less important position on 

 Life's Stage, and then die out completely. 



Of such are the once abundant Graptolites, which had their 

 beginning in the Cambrian, their maximum in the Ordovician and 

 Silurian, and then disappeared. 



The Trilobites, which began in the Cambrian, attained their 

 maximum in the Silurian, lived on into Carboniferous times, and 

 then disappeared. 



The Merostomata {Pteryijotus, Eurypterus, Stylonurus, &c.) 

 began in the Silurian, attained their maximum, lived on into the 

 Devonian and Carboniferous periods, and then became extinct. 



Persistent Gt oups. 



A gain we have persistent forms of which we seem to see neither 

 the beginning nor the ending. 



Of these we may name the Protozoa, embracing the Badio- 

 laria and the Foraminifera, both persistent in rocks of all ages 

 and well represented at the present day. 



The Porifera (Sponges) which, though materially differentiated 

 in the course of geological ages, have lived on till to-day. 



The Crinoidea (Sea-lilies), represented from Silurian times to 

 the present day, but not nearly so abundant as in Palaeozoic times. 



The Starfishes (Asteroidea and Ophiuroidea), both persis- 

 tent types from Silurian (or earlier) times to the present. 



The Annelida again are met with in all strata and also living. 



The Brachiopoda, beginning in the Cambrian, enormously 

 developed in Silurian, Devonian, Carboniferous, and Secondary 

 deposits, and still surviving in diminished numbers in modern seas. 



Mollusca, represented in past iime by the persistence of 



Lamellibranchiata ..... Cambrian to recent. 



Sraphopoda ... . . Cambrian ? to recent. 



ChitonidsB ...... Siluriau to recent. 



Pteropoda ...... Cretaceous to recent. 



Prosobranchiata ..... Cambiian? to recent. 



Cephalopoda (in part; .... Ordovician to recent. 



Pulmonifera (Zonites and Pupa) . . Coal-measures to reot nt. 



