996 PRINCIPLES OF STRATIGRAPHY 



animals. The skeletal portions of the dead plankton often accumu- 

 late in vast quantities on the bottom, and, in the greater depths 

 where terrigenous sediments are absent, they usually form diatoma- 

 ceous, radiolarian, globigerina, pteropod, and other oozes. The 

 purity of such oozes, i. e., their freedom from clastic sediment, is 

 usually an index of the purity of the water in which they were 

 deposited, but from this we cannot always decide that such oozes, 

 when found in fossil state, indicate deep sea. The absence of 

 clastic sediment may be due to the low relief of the land, which 

 may have been worn down to base-level, thus allowing water of 

 moderate depth near shore to be free from detrital material. 



B. Halonekton. 



The term nekton, derived from the Greek vt^ktos, which means 

 swimming, was introduced by Haeckel in 1890 (4), for those ani- 

 mals which lead an actively swimming life. The group is typified by 

 the class of fishes. A torpedo-like form, terminating anteriorly 

 in a head, and perfect bilateral symmetry are the chief character- 

 istics of these animals. A strong musculature for propulsion is 

 usually situated in the posterior portion of the body, while ap- 

 pendages for balancing and steering are also usually present. The 

 body is non-transparent and a calcareous supporting skeleton is 

 ordinarily developed. Typical nektonic animals of the modern sea 

 are : the squids, the fish, and the degenerate mammals — whales, 

 porpoises, etc. Besides holonektonic forms there are in the sea 

 epinektonic ones, /. c, sessile forms more or less pemanently at- 

 tached to a swimming host. The whale barnacle, attached to the 

 under side of whales, and the ship's barnacles are examples among 

 invertebrates, while the Pilot-fish attached by suckers to sharks 

 represent this type among vertebrates. 



C. Halobenthos. 



The term benthos, from (ih/do's, the depths of the sea, was like- 

 wise introduced by Haeckel in 1890 (4). It covers those organisms 

 which inhabit the sea-bottom. We may divide the benthos into 

 sedentary and vagrant (vagile) benthos, the former attached to 

 the bottom, the latter moving over it. Living in such intimate rela- 

 tion with the sea-bottom, halobenthonic organisms are to a high 

 degree dependent upon its facies, and their remains are generally 

 entombed in the region where they have lived, instead of being 



