Brachiopod fossils vary greatly in size, from less than one-fourth of 

 an inch to several inches. They are of calcium carbonate or chitino- 

 phosphatic composition. Many of the shells have variously shaped structures 

 or platforms which supported soft parts of the animal; they are found on 

 the inside of the valves. 



Shells of brachiopods are characterized by a wide diversity of appear- 

 ance. Some are circular or elliptical in outline while others are irregular. 

 Some shells have two very convex valves; some have one convex and one 

 almost flat valve; others may have one convex and one concave valve. 



Where the halves of the shell are joined, one valve typically has a 

 pointed beak. At this end of the fossil is the hinge line, an area which 

 varies greatly in different kinds of brachiopods. When the hinge line is 

 short, the shell shape is pointed at that end. 



A shell with a wide hinge line may possess greater width than length. 

 An example is Mucrospirijer mucronatus. This species is characterized by 

 lines radiating from the center of the hinge line and by a prominent center 

 fold in one valve and corresponding depression in the other. 



Some brachiopod shells are smooth; others, such as the common Atrypa 

 reticularis, display prominent ridges and grooves radiating out from the 

 hinge of the shells plus growth lines concentrically arranged. 



Mollusks 

 Gastropods 



Snails, or gastropods, are mollusks possessing a single shell which is 

 usually coiled into a spiral, although it may be conical or saucer-shaped. 

 The animal lives in the entire shell which is not partitioned into chambers. 

 Growth lines are often present encircling the shell. Ornamentation of 

 various types may occur including ridges, folds, and spines. 



Although gastropods were abundant in the ancient seas the gastropod 

 shells were quite easily destroyed because they were mainly composed of 

 a relatively soluble mineral called aragonite. Many gastropod fossils may 

 be merely fillings of the shell interior. They are internal molds which do 

 not indicate the outer form of the shells and therefore are difficult to 

 identify. 



Some gastropod fossils may be recognized because of characteristic 

 coiled or conical forms. Others, such as Platyceras, possess horn-shaped 

 shells of irregular growth reflecting irregularities in the object to which the 

 shell was fixed during life. 



Peiecypods 



Clams, or pelecypods, have calcium carbonate shells composed of two 

 valves typically equal in size and shape, each half being the mirror image 

 of the other. Each valve is often asymmetrical in outline. These features 

 are the most evident means of differentiating pelecypods from the more 

 commonly found brachiopod fossils which also possess two valves but of 

 unequal size and shape. 



Concentrically arranged growth lines are commonly present as are 

 various types of ornamentation such as radiating ribs. Fossils may consist 



