ARTIFICIAL STRUCTURES; COPROLITES 1093 



3. Artificial Structures. 



Beginning with the worm-tubes already mentioned, or even with 

 the excavations made by some animals, we have this type of fossil 

 increasing in importance as we rise in the scale of organic being. 

 Even as far down as the group of rhizopods we find many types 

 building shells by cementing foreign particles with the aid of a 

 secretion. This type of habitation is analogous to the worm-tube 

 already mentioned. Though represented in most classes of animals, 

 it is not until we reach man that these artificial structures assume 

 any great importance. 



Thus the implements of stone, shell, bone or metal, the pottery 

 and the copper, bronze and iron vessels ; the beads and other orna- 

 ments; the coins and the habitations of man from the rude excava- 

 tion in the rock to the buried cities of historic time, with all their ac- 

 cessories, belong to this type of fossils. This group, therefore, falls 

 largely in the province of Anthropology, or the science which is 

 concerned with man in all his relations, including his palaeontology. 



4. Coprolites. 



The excrements of certain animals have a definite and recogniz- 

 able form, and so become valuable indices to the former presence 

 of such animals. Most important among these are the coprolites 

 of fishes and reptiles, the latter constituting important fossils in 

 the Mesozoic rocks. Very much concerning the food of the ani- 

 mal can often be learned from the remains found within the copro- 

 lite. The excrements or "castings" of worms also belong here. 

 They generally consist of cord-like masses of molded sand which 

 have passed through the intestine of the worm and from which 

 the nutrient organic matter has been abstracted. They cover some 

 modern beaches in great quantities, and are not infrequently pre- 

 served. Certain echinoderms, particularly holothurians, have rec- 

 ognizable excrem,ents. Rothplotz has found an abundance of 

 calcareous rods in the bottom deposits of Great Salt Lake, which 

 he regards as excrements of Artemia, an abundantly represented 

 crustacean in this body of water. They closely resemble known 

 excrements of Artemia, but are calcareous, since the species of the 

 Salt Lake are supposed to feed on calcareous algai. 



