CRUSTACEA. OSTRACODA 315 



middle) towards the ventral border. Surface generally striated. 

 Anterior margin notched for the passage of the antenna?. Ordo- 

 vician to Carboniferous. Ex. E. tuberosa, Silurian. 



Cy there. Shell oblong-ovate or subquadrate, highest in front ; 

 smooth or ornamented with pits, spines, or ridges. Hinge with 

 teeth anteriorly and posteriorly. Ordovician to present day (chiefly 

 Cretaceous and later). Ex. C. striato-punctata, Eocene; C. punctata, 

 Pliocene. 



Cypris (fig. 135). Carapace thin, smooth or punctate, kidney- 

 shaped or oval ; ventral edge often concave. Left valve the larger. 

 Hinge without teeth. Purbeck Beds to present day. Fresh water. 

 Ex. C. faba, Miocene ; C. gibba, Oligocene to present day. 



Cypridea. Valves ovate-oblong, convex in the middle, broad 

 at the anterior third, narrower behind ; with a notch at the anterior 

 ventral angle behind a beak-like process. Surface smooth, punctate, 

 or tuberculate. Hinge-margin straight, along the middle third of 

 the dorsal edge. Left valve the larger. Purbeck, Wealden, and 

 Oligocene. Fresh water. Ex. C. valdensis, Wealden Beds, etc. 



Distribution of the Ostracoda 



The Ostracods have a very wide distribution at the 

 present day ; many forms are marine, and some are 

 abundant in fresh water. The marine forms often occur 

 in shoals ; some are pelagic, but others live on the sea- 

 floor and are more abundant in shallow than in deep 

 water, only fifty-two species being found beyond the 500 

 fathom line. 



The fossil forms are very numerous, the earliest occur- 

 ring in the Cambrian. Leperditia, Primitia, and Beyrichia 

 are abundant in the Lower Palaeozoic ; Entomis in the 

 Devonian ; and Cypridina and Bairdia in the Carboni- 

 ferous. Cypridea is common in the Purbeck and Wealden 

 Beds ; and Cythere in the Tertiary formations. 



