BIOLOGICAL PALAEONTOLOGY 107 



amount of enrolment was developed in that conservative 

 group as early as the Carboniferous (while strong involu- 

 tion has characterized Nautilus since Jurassic times), the 

 septa usually retain great simplicity. Not until the 

 Cainozoic do corrugations comparable with those of 

 Devonian "Goniatites" affect the Nautiloid septum, 

 and even this small degree of complication has been 

 abandoned by living forms. The different speed of 

 evolution in these presumably homogenetic classes is 

 associated with important results from a stratigraphical 

 point of view. The Ammonoids appear in the Devonian 

 and disappear in the Cretaceous, attaining, in their rela- 

 tively brief career, acmaic prominence of an extreme 

 type. The Nautiloids arose in the Cambrian (or before), 

 and are still living, but they cannot compare with their 

 more progressive relatives in specific variety or numerical 

 abundance. Their deliberate and unaspiring progress 

 may be correlated with their early introduction. It is 

 usual for the less-specialized, ancestral stocks of groups 

 that have produced progressive branches to persist long 

 after their precocious offspring have fallen from the 

 acmaic heights. The Atrematous Brachiopods are 

 represented to-day by species of Lingula remarkably 

 like many of those prevalent in Ordovician seas; but 

 the Orthacea, Strophomenacea and Spiriferacea, groups 

 which seem certainly to have had an Atrematous 

 ancestry, have had their day, and quite, or almost, 

 ceased to be. The Echinoid family of the Cidaridae 

 is represented by closely similar forms in the Lias and 

 the Holocene, but many of the Jurassic and Cretaceous 

 types, developed from the Cidarid stock and of far 

 greater moment in their time, have been long extinct. 

 Almost every phylum can show its conservatives and 

 progressives; while the practically static qualities of 

 those Protozoa whose palaeontological history can be 



