A Suggestion on Extinction. 469 



Taxocrinus, indeed, have almost the same range. This group 

 appears to have some very generalised characters in com- 

 parison with its Palaeozoic relations, and seems, moreover, to 

 be in some measure related to the later groups of Crinoids 

 which came into existence in the Mesozoic, and have per- 

 sisted to the present time. Amongst these later forms 

 Fentacrinus, Extracrinus, and Antedon have persisted since 

 the beginning of the Mesozoic with very little change. 



Of the Echinoids, typically regular forms like Cidaris have 

 existed ever since the Trias. The suddenly produced large 

 numbers of irregular forms, like Echinocorys in the Cretaceous, 

 was followed by a very quick extinction of many of the 

 genera, though both regular and irregular forms have con- 

 tinued in abundance to the present time. 



Fossil Polyzoa are found plentifully throughout all the 

 deposits as far back as the Ordovician. Some of the simpler 

 genera of the Cyclostomata, as Stomatopora and Berinicea, 

 have ranged throughout the whole of that time, and a great 

 many living genera of the same division range far back into 

 the Mesozoic. The peculiar group of Monticuliporoids, 

 which some consider as related to the Polyzoa, and which 

 acquired such dominance in Ordovician and Silurian times, 

 and underwent such variety in form and structure, doubtfully 

 survived the Palaeozoic. The same range is found in the 

 specialised group of the Cryptostomata which contains the 

 bulk of the Palaeozoic Polyzoa like Fenestella, Folypora, 

 Fhahdomeson, and their allies. The Chilostomata, which 

 form the bulk of living Polyzoa, only date back to the 

 Jurassic. 



The Brachiopoda, like the Crinoids, showed their maxi- 

 mum development in Palaeozoic times. The great abundance 

 and extraordinary specialisation of forms of FroductnSy 

 SpirifeVy Pentamems, Cyrtia, Merista, Uncites, and Stringo- 

 cephalus, as well as their variety in shape and size and in 

 the condition of their brachial supports, is quite in accord- 

 ance with the comparatively short range of these genera, and 

 especially of the more remarkable species. The long-winged 

 forms oiSpirifer became extremely dominant in the Devonian, 

 but underwent as rapid an extinction. The simple Spirifer 



