NOTES ON THE BRITISH JURASSIC BRACHIOPODA. 19 



the geological periods ; from the oldest deposits known to 

 contain vestiges of animal life up to the present time. 



So prolific were they during the Silurian epoch that 

 that period is often described as "The age of brachio- 

 pods." 



As a class the Brachiopoda certainly appear to be dying 

 out. Many of the forms which existed daring the de- 

 position of the Palseozoic and Mesozoic rocks are now 

 quite extinct, such as the well known genera Athyris, 

 Atrypa, Chonetes, Lingulella, Orthis, Pentamerus, Prodiictus, 

 Eetzia, Spirifer, Spiriferina, Strophoviena, and others. Some, 

 however, had a great range in time. Snch, for example, 

 are the genera Lingula, Crania and Discina, all of which 

 have persisted, with slight modifications in form, from the 

 earliest epoch represented by fossiliferous rocks up to the 

 present time. 



Davidson, in 1884, enumerated some 74 genera, about 

 887 species, and 80 named varieties, which he figured in his 

 monograph as occurring in the British rocks (10, p. 394). 



This is by no means an exaggerated estimate of the 

 British species, neither does it give us anything like an 

 approximate idea of the whole number of species known 

 to have existed on the globe, between three and four thou- 

 sand having already been described or figured by various 

 palaeontologists in different parts of the world. 



The number of brachiopods living at the present time, 

 so far as genera and species are concerned, is very limited 

 indeed ; roughly speaking, about 22 genera and subgenera, 

 and about 99 named species, and 29 uncertain ones (11, 

 p. 4). . _ 



Judging by what we know of the rocks in which they 

 are entombed, the organic remains associated with them, 

 and the habitats of many of the recent forms, we may 



