302 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The modern species of Estheria live in fresh water, especially 

 in ponds and rain pools, very rarely in brackish water. The 

 concentric ridges, coarsest near the umbo, are due to the fact 



Fig. 133. — Estheria helfragei Packard, from Texas. (X 5.) A, the body of the 

 animal shown after the removal of one valve. a«/., antenna ; e., eye; w., adductor 

 muscle. B, exterior of one valve. (Redrawn from Packard.) 



that '' the integument of the outer surface of the shell is not 

 cast off in ecdysis [molting], but remains in position, giving rise 

 to a series of lines of growth marking the increased size of the 

 shell at each molt" {^li). The spaces between the suc- 

 cessive ridges are marked with irregular, 

 interlacing striae, differing thus from any 

 pelecypod shell which the bivalved carapace 

 otherwise closely resembles. 



E. ovata is abundant in the Triassic of 

 North America, such as the Newark beds 

 of the Atlantic coast and the Shinarump 

 of Arizona. 



Fig. 134. — The mi- 

 nute phyllopod crus- 

 tacean, Estheria 

 ovata Lea, from the 

 fresh-water Shina- 

 rump formation of 

 southern Utah. ( X 

 12 ; line below indi- 

 cates true size.) 



1. Sketch the outside of one valve. 



2. How much of the body of the animal 



did the shell cover ? 



3. How can you tell from the fossil alone 

 that this is a crustacean and not a pelecypod ? 



4. What is the origin of the concentric ridges ? 



5. Name three characters which distinguish crustaceans. 



