CRUSTACEA. OSTRACODA. 187 



Estheria. Valves thin, horny ; ovate, oblong or quad- 

 rilateral, united at the straight dorsal border, and with 

 the apices placed anteriorly or nearly central. Surface 

 generally covered with concentric stria?. Devonian to 

 present day. Living in fresh or brackish water. 



OBDER. OSTRACODA. 



The Ostracods are generally minute. The body is not 

 segmented and is usually compressed laterally and com- 

 pletely enclosed in a bivalved shell (or carapace), which 

 may be horny or calcareous ; one valve is placed on each 

 side of the animal. Dorsally the two valves are joined 

 together by means of an elastic ligament which serves to 

 open the shell ; an adductor muscle passes from the 

 interior of one valve to the other. There are seven pairs 

 of appendages which can be protruded when the shell is 

 opened. In the marine forms the shell is notched an- 

 teriorly so as to allow the antenna? to pass through when 

 the shell is closed. The head carries two pairs of antennae, 

 one of mandibles, two of maxilte. The thorax has two or 

 three pairs of appendages, which are not leaf-like. The 

 abdomen is rudimentary and is without appendages; it 

 terminates either in a fork or a spiny plate. Respiration 

 takes place by means of the general surface of the body. 

 The carapace is in almost all cases the only part which 

 occurs fossil ; its surface may be smooth or variously 

 ornamented. 



Leperditia. Carapace smooth, convex, elongated, a 

 little wider posteriorly. The right valve larger than the 

 left. Hinge-line straight. There is a small tubercle 

 (eye-spot) placed anteriorly near the hinge ; and posterior 

 to it is a slightly elevated circular area. Ordovician to 

 Carboniferous. 



