RECENT BRACHIOPODA 267 



this is more than 60 per cent of the living genera. Two have lived since 

 the Ordovicic (Lingula and Crania), 6 since the Jurassic (Acanthothyris, 

 Eucalathis, Magellania, Megathyris, Terebratella, and Terebratulina), 4 

 since the Cretaceous (Agulhasia, Cistella, PDiscinisca, and Thecidium), 

 1 since the Miocene (Platidia), 7 since the Pliocene (Glottidia, Hemi- 

 thyris, Dyscolia, Liothyrina, Macandrevia, Terebratalia, and Miihlfeld- 

 tia), and 3 since the Pleistocene (Bouchardia, Dallina, and Gwynia). 

 Of these 23 genera 6 have not spread into water as deep as 1,000 feet, 

 these being Agulhasia, Bouchardia, Discinisca, Lingula, Glottidia, and 

 Megathyris. Of genera that have spread beyond this depth, but which 

 still have their best development in waters less than 500 feet, are Gwynia, 

 Magellania, Terebratalia, and Terebratella. The genera having fossil 

 representation, with their best development in the present seas at depths 

 greater than 500 feet, are Acanthothyris, Cistella, Crania, Dallina, Dys- 

 colia, Eucalathis, Hemithyris, Liothyrina, Macandrevia, Miihlfeldtia, 

 Platidia, Terebratulina, and Thecidium. All of these are of long endur- 

 ing stocks that had their rise at least as early as the Jurassic, and if we 

 add to these the other truly deep-sea brachiopods, also of ancient phyla, 

 but not known to have fossil representation, such as Basiliola, Cryptopora 

 (both rhynchonellids), -Chlidonophora (primitive terebratulid) , Frieleia, 

 and Pelagodiscus (discinid), we can say that the present deep-sea forms 

 as a rule did not begin to migrate to this habitat earlier than the middle 

 Mesozoic, and, further, that this adaptation is still going on. The truly 

 abyssal forms, as Basiliola, Chlidonophora, Frieleia, and Pelagodiscus, 

 are probably of stocks even older than the middle Mesozoic, and these 

 genera may have begun their abyssal march as early as the beginning of 

 the Mesozoic. It is, however, a noteworthy fact that of the great multi- 

 tude of Paleozoic genera not one is known to have become wholly abyssal 

 LQ its habitat; on the contrary, the two oldest Paleozoic genera that are 

 still alive have not gotten far away from the strand-line. Lingula is still 

 restricted to the littoral and Crania, whUe as a rule now a deeper water 

 genus, is by no means restricted to the abyss, although it has been taken 

 at 818 fathoms. These observations lead to the conclusion that the oceans 

 probably did not begin to get exceedingly deep until after the great Ap- 

 palachian Eevolution toward the close of the Paleozoic — that is, early in 

 the Mesozoic — and that this deepening has been going on since then. 

 These views are also in harmony with the conclusion attained by Wal- 

 ther* from a study of the life of the present deep seas. 



*Walther: Origin and peopling of the deep sea, Amer. Jour. Sci. (4), vol. xxxi, 1911, 

 pp. 55-64. 



XIX— Bull. Gbol. Soc. Am., Vol. 22, 1910 



