990 PRINCIPLES OF STRATIGRAPHY 



pterosaurs among reptiles, the birds, and, among mammals, the 

 bats. 



III. The Abyssal Life District. 



A. Marine. The marine abyssal districts comprise the lightless 

 depths, or generally those depths exceeding two hundred fathoms. 

 (Chapter III.) As assimilating plant life is absent in these dis- 

 tricts the food supply of the organisms existing in them must 



' be wholly derived from the districts in which such assimilating 

 vegetation exists. A large proportion of the food of the abyssal 

 animals is contained in the organic oozes and sediments which con- 

 stantly settle down in a more or less decomposed state from the 

 lighted districts. The abysso-pelagic district is frequently invaded 

 by organisms from the pelagic district, which descend into the dark 

 regions during the day. 



B. Fresh Water and C. Terrestrial. In the non-marine realms 

 the lightless districts are sparingly represented. Lakes of great 

 depth, whose bottom is in perpetual darkness, do exist, but the life 

 of these dark regions is mostly unknown. Cave life, on the other 

 hand, both fluvio-lacustrine and terrestrial, has been made the sub- 

 ject of much study, and its characteristics are known to harmonize 

 with the peculiar conditions which give these districts their especial 

 stamp. 



BIBLIOGRAPHY XXVI. 



1. CHAMEERLIN, THOMAS C. •1900. The Habitat of the Early Verte- 



brates. Journal of Geology, Vol. VIII, pp. 400-412. 



2. GRABAU, A. W. 1899. The Relation of Marine Bionomy to Stratigraphy. 



Chapter III in the Geology and Palaeontology of Eighteen-mile Creek. 

 Bulletin of the Buffalo Society of Natural Sciences, Vol. VI, pp. 319-365- 



3. GRABAU, A. W. 1913. Ancient Delta Deposits of North America. Bul- 



letin of the Geological Society of North America, Vol. XXIV pp. 498-526. 



4. LOVEN, SVEN. 1883. On Pourtalasia, a Genus of Echinoidea. Stock- 



holm. 



5. O'CONNELL, MARJORIE. 1912. The Habitat of the Eurypterida. 



Paper presented before the New York Academy of Sciences. Also in 

 Grabau (2), pp. 499-515- 



6. SCHIMPER, A. F. W. 1898. Pflanzengeographie auf physiologischer 



Grundlage. Jena. English translation by W. R. Fisher. Plant Geog- 

 raphy, Oxford, 1903. 



7. SEMPER, KARL. 1881. Animal Life as Affected by the Natural Condi- 



tions of Existence. Appleton International Scientific Series, Vol. XXX. 



