THE MAIN KINDS OF FOSSILS 



37 



mail shells) and the Scaphopoda (tusk-shells) are unimportant as fossils. The other three 

 classes are discussed separately. 



CLASS PELECYPODA. 



These are the clams, oysters, 

 Typically, the body is en- 



beak 



hinge 



rear 



scallops, and their relatives, 



closed in a shell of two valves, like the brachiopod's, but 

 in the clam the valves are equal but not bilaterally symmet- 

 rical (fig. 36). The outline of the shell varies greatly. Some 

 are almost round, others are much longer than high, others 

 have one or two wing-like projections (see fig. 37) and are 

 referred to as "winged" shells. The valves are locked together 

 by projections of the hinge which are called teeth. The small 

 teeth under the beak are called cardinals and the long teeth 

 parallel to the hinge are called laterals (see fig. 38). The 



animal pulls the valves shut by means of a pair of muscles. Fig. 36 



The muscles are attached to definite areas of the shell which 



are called muscle scars (see fig. 38). The front one is the anterior muscle scar and the 

 one is the posterior muscle scar. 



Most pelecypods crawl on the bottom of the sea, rivers, and 

 lakes and move with their fleshy foot. A few kinds are able to swim by 

 clapping their valves open and shut; the scallop is one of these swim- 

 ming pelecypods. Still others live all their adult lives attached to the 

 bottom either by silken threads or directly by cementing one shell to a 

 solid object; the oyster is one of these attached pelecypods. Most of 

 the pelecypods are marine but a few groups live in fresh water. Pele- 

 cypods are known from the Ordovician to the present. 



Fig. 37 

 The Paleozoic rocks of Ohio have yielded many genera and species 

 of pelecypods but they are usually scarce and poorly preserved. One exception to this rule is 

 the excellent preservation of pelecypods in the Logan formation (Mississippian). 



CLASS GASTROPODA. The snails usually 

 have a spirally coiled shell (fig. 39) which is 

 typically carried on the back of the animal. 

 When the animal is disturbed, it can withdraw 

 completely into the shell and in some forms the 

 shell is then closed by a horny or limy lid, the 



operculum . The spire 

 may be coiled in one 

 plane or it may coil 

 downward after the first 

 whorls and form a pointed, 

 high-spired shell. Loosely 

 coiled shells have a cen- 



cardinal teeth 



lateral teeth 



Fig. 38 



muscle scars 



aperture 



Fig. 39 



tral depression, the umbilicus (fig. 40), in the base of the whorls. Gas- 

 tropods are found in the sea, in fresh water, and on land. Their geologic 

 range is Cambrian to present. 



Fossil gastropods are found in all the ex- 

 posed rocks of Ohio, including the Pleistocene 

 lake, pond, and river sediments. In the Paleo- 

 zoic rocks the shell is usually poorly preserved 

 but there are exceptions in which the ornamen- 

 tation of the shell is beautifully sharp and clear. 



umbilicus 



Fig. 40 



