FOSSIL INVERTEBRATES AND PLANTS 101 



modern corals have skeletons of varying form, shaped like a 

 brain, staghorn, or flower. 



Echinoderms — star-shaped (starfishes), liliform (crinoids) (figs. 

 56 and 57), bun- or biscuit-shaped (sea urchin) (fig. 58) covered 

 by a skeleton of calcite plates or ossicles that show the cleavage 

 of the mineral calcite when broken. 



Worms — seldom found as actual fossils but as traces in the 

 form of burrows, tubular shells, or tracks. 



Bryozoa — branching, lacy, or massive coralline skeletons con- 

 taining tiny pores in which the colonial animals lodged (fig. 59). 



Brachiopods (fig. 60) — with two smooth or ribbed shells of un- 

 equal size, the larger one with a hole at the beak. Right and left 

 sides of the shells are identical (bilaterally symmetrical). 



Mollusks — clams (fig. 61) have two nearly equal shells in each 

 of which the right and left sides are unequal. Snails (fig. 62) 

 have a single shell commonly coiled spirally, rarely coiled in one 

 plane. Cephalopod shells coil in a plane (rarely spiral) or are 

 straight and are divided into many chambers by transverse par- 

 titions or septa. Cephalopods in which the trace of the septa on 

 the side wall is a simple curve are called nautiloids (fig. 63) and 

 those with complicated septa, ammonoids (fig. 64). 



Arthropods — invertebrates with jointed legs such as insects, 

 crabs, lobsters, and spiders. The commonest fossil arthropods are 

 trilobites (fig. 65) which have a segmented body, head, and tail, 

 characterized by a more or less elevated central axis. 



Locating jossil-b earing ledges. — The accurate location of the exact 

 layer from which a collection of fossils is taken is most important. 

 Loose or drift fossils, or specimens lacking essential information 

 as to their locality, may have biological importance but will be 



