188 CRUSTACEA. OSTRACODA. 



Primitia. Carapace equi valve, convex, oblong. Hinge - 

 line straight. Each valve has a dorsal groove which 

 starts from the hinge-line. Cambrian to Carboniferous. 



Beyrichia. Carapace elongated, inflated; dorsal 

 border straight, ventral border semicircular. On the 

 surface there are two or three large furrows which pass 

 from the dorsal towards the ventral edge, the parts be- 

 tween being convex and often tuberculate. Cambrian to 

 Devonian. 



Cypris. Carapace thin, smooth or punctate, kidney- 

 shaped or oval ; ventral edge often concave. Oligocene 

 to present day. Freshwater. 



Cypridea. Differs from Cypris in having a beak-like 

 process at the anterior ventral angle. Purbeck and 

 Wealden. 



Distribution of the Ostracoda. 



The Ostracods have a very wide distribution at the 

 present day ; they are mainly marine, and often occur in 

 shoals. They are most abundant in shallow water, only 

 fifty-two species being found below the 500 fathom line. 

 The fossil forms are very numerous, the earliest occurring 

 in the Cambrian. 



ORDER. CIRRIPEDIA. 



The Cirripeds include the barnacles, acorn-shells, etc. ; 

 forms which differ considerably in appearance from the 

 other crustaceans, and which were for a long time regarded 

 as mollusks. The body is completely enclosed in a fold of 

 the integument, which commonly forms a calcareous shell. 

 The animal, in the adult state, is fixed to a foreign object 

 by the anterior end of the head, either directly or by 



