DISTEIBUTIOlir OF BKACHIOPODA. 255 



before their final, or even approximate, extinction will have been 

 reached. Indeed, if specific variation be considered a just criterion 

 in the determination of the question of development or extinction, 

 it may be reasonably doubted whether we are not now actually 

 more remote from the apparent closing period of the existence of 

 this group of animals than we were at the beginning of the Tertiary 

 epoch, possibly a million or more years ago. Some sixty species 

 and varieties, referable to ten genera, of brachiopods have been 

 described by Davidson (1870) from the Tertiary deposits of Italy, 

 and these constitute by far the largest number furnished by any one 

 country of all the Tertiary forms that have thus far been described. 

 England has but seven species, and France and Germany scarcely 

 more, while the United States have not even as many. The num- 

 ber of distinct forms found in the present Mediterranean waters is 

 about fifteen, or only one-fourth the number found in the Italian 

 Tertiaries ; but it must be recollected that the periods of time which 

 we are here comparing are of very unequal dm'ation, the " recent " 

 period being only a mere figment of that indicated by the Tertiary 

 formations. If to the recent and Quaternary species we add those 

 found in the Upper Pliocene we will have a total of twenty-one 

 species, which will then considerably outnumber the species from 

 the entire Eocene series (thirteen or seventeen), and only fall eight 

 or nine short of the total number from the combined Lower, Middle, 

 and Upper Miocene. As to generic development, we find the Italian 

 Tertiary species to belong to ten genera or sub-genera, to wit : Tere- 

 bratula, Terebratulina, Waldheimia, Terebratella, Megerlia, Platy- 

 dia, Argiope, Thecidium, Rhynchonella, and Crania, all of which, 

 except Terebratella and Rhynchonella, are still found in the Medi- 

 terranean waters.* The number of Tertiary brachiopod genera and 

 sub-genera thus far recognised is fourteen," as against twenty or 

 more of the present day. 



The earliest known Brachiopoda, or those of the Cambrian pe- 

 riod, belong almost exclusively to the group of the Brachiopoda 

 inarticulata (Pleuropygia), forms in which the shell is horny-calca- 

 reous and devoid of a dental articulation — Lingula, Lingulella, Lin- 

 gulepis, Obolella, Kutorgina, Acrothele, Acrotreta, Discina. The 

 remains of Orthis (and Orthisina ?) alone of the Brachiopoda articu- 



* Thecidium Mediterranoum appears to belong only to the African side of 

 the MediteiTanoan. 



