274 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



the Devonian; the fresh-water genus Cypris is unknown prior to 

 the Carboniferous. The species of the first two genera, whose dis- 

 tribution is almost universal, are particularly numerous in the Creta- 

 ceous and Tertiary deposits, and present some remarkable instances 

 of reputed longevity. Two recent species of Bairdia, B. sub- 

 deltoides and B. augusta, are claimed by Gerstaecker 10 to be 

 found also in the Carboniferous deposits, but it is by no means 

 improbable that the identification rests on improper determinations. 

 This is rendered the more likely, seeing how very few, compara- 

 tively, are the recent forms that have been identified as occurring 

 in the Tertiary deposits. Of some seventy species or varieties of 

 Entomostraca described by Rupert Jones from the British Tertiary 

 deposits from the Eocene to the Pliocene inclusive, not more than 

 seventeen or eighteen are recognised by him as constituting a part 

 of the recent fauna. A re-examination of the forms may possibly 

 increase this number somewhat, but it is certainly very remarkable 

 that of two hundred and twenty or more species of Ostracoda 

 dredged by the officers of the "Challenger" expedition only three 

 or four had previously been described by paleontologists. While 

 many of the modern species have a broad distribution, the number 

 of forms which are known to be in any way cosmopolitan is ex- 

 ceedingly limited (species of Halocypris and Cythere). On the other 

 hand, there are some very marked instances of antipodal reappear- 

 ance. Several well-known British and northern forms have been 

 identified from Kerguelen Island and other remote regions of the 

 Earth's surface. Although apparently penetrating to the profound- 

 est depths of the sea, the number of both species and individuals 

 rapidly diminishes beyond a comparatively shallow superficial zone. 

 Only fifty-two species were obtained by the "Challenger" from 

 a depth exceeding five hundred fathoms, and but nineteen from 

 below fifteen hundred. The much greater diversity of the shore 

 fauna as compared with that of the open sea is shown by the fact 

 that among the ' ' Challenger " Ostracoda only twenty-eight genera 

 were represented, whereas on the British coast alone there are at 

 least thirty-one. Among the oldest known representatives of the 

 order are the Primitia prima and Leperditia Cambrensis, from the 

 St. David's (Lower Cambrian) rocks of Wales. 



The remains of decapod crustaceans in the Paleozoic rocks are 

 exceedingly scanty, as indeed they are also in the earlier part of 



