304 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Leperditia (Fig. 135). Cambrian- Devonian. 



Shell thick, smooth, biconvex, 2 mm. -2 2 mm. long. Hinge 



line straight, usually angular at the extremities. Anteriorly, 



near the hinge, a small tubercle 

 ^/^.^^r,^ ri marks the position of the eye. 



1. Sketch shell (a) view of 

 one valve ; (b) hinge view of 

 combined valves. Label eye 

 tubercle, ligament. 



2. How much of the body of 

 the animal did the shell cover ? 



3. Why is this bivalve shell 

 placed under the Crustacea and 

 not under the Pelecypoda ? 

 How can this be told from 

 the fossil alone ? 



4. Compare the shell of 

 Leperditia with that of Estheria (a phyllopod). How do they 

 differ ? 



5. What opens the valves? 



Fig. 135. — A fossil ostracod, Leperditia 

 alta Conrad, from the uppermost 

 Silurian of New York. The ends are 

 imperfect. A, left valve (//.) of the 

 carapace, showing the incurving 

 edges of the larger right valve 

 {rt.), and the eye tubercle {eye), 

 beneath which the eye was located. 

 B, view of hinge margin. (Both X 2.) 



What closes them ? 



Sub-class 4, Copepoda 



Body elongate, segmented, covered with a carapace ; thorax 

 usually with four or five pairs of biramous feet ; abdomen free 

 from appendages. Copepods are mostly marine though many 

 inhabit fresh w^ater. Very many are parasitic, especially on 

 fish ; few fish are entirely free from them. Name from Greek 

 cope, oar, + pous {pod), foot. So named because their feet are 

 used mostly for swimming. Unknown in the fossil state. 



Two kinds of copepods occur, — {a) parasitic, e.g. carp lice 

 (Argulus) ; {b) free-swdmming, e.g. water fleas (Cyclops). Under 

 favorable conditions one cyclops might have five billion descend- 

 ants in one year. In this form anal respiration is especially 

 noticeable. This method of breathing, common among the 

 lower Crustacea, consists of rhythmical contractions of the intes- 

 tine, which admit and expel the water. 



