OSTRACODA. 505 



there may be no metamorphosis. Parthenogenesis is a not uncom- 

 mon phenomenon in the Ostracodes. 



The Ostracoda, often called "Water-fleas," are represented by 

 very numerous forms both in fresh water and in the sea. The com- 

 monest fresh-water types are the little Cyprides (fig. 360, b). The 

 marine Ostracodes {Cy there, Cypridina, &c.) 3 are mostly shallow- 

 water forms, and are of small size ; but there are deep-sea types 

 which attain comparatively gigantic dimensions (nearly an inch in 

 length). Numerous fossil forms of the Ostracodes are known, their 

 remains occurring in all formations, from the Cambrian onward. 



It is only the carapace-valves of the Ostracode Crustaceans that 

 are preserved in the fossil condition, with the rarest exceptions ; and 

 the general form of the carapace is often very similar in different 

 genera. Hence the palaeontologist has to rely, in the discrimination 

 of these minute fossils, upon small variations of shape, differences 

 in the thickness of the valves, the characters of the edges of the 

 valves, or the manner in which they are hinged to one another, or, 

 lastly, the surface-ornamentation. Besides the difficulty attaching to 

 the study of the fossil Ostracoda from their small size and general 

 similarity of appearance, it is often by no means easy to distinguish 

 between the cephalic and the posterior extremity of the body. 

 When not alike, the most contracted extremity is to be regarded as 

 the head, and the widest as the hinder end of the carapace. The 

 former, as a rule, carries grooves or tubercles, when such structures 

 are present at all. The tubercles of the test, where developed, ap- 

 pear to represent the eye ; and the grooves and intervening lobes, 

 which are found in many forms, have been aptly compared by Bar- 

 rande to the furrows and lobes of the glabella of Trilobites. There 

 are many types, however, in which there are no conspicuous external 

 markings, and in which the two ends of the carapace are similar. 

 The Mesozoic and Tertiary Ostracoda are very small, and the same 

 is true of a large number of Palaeozoic species ; but among the 

 latter we find some comparatively large types, which, like Leperditia 

 Hisingeri, may reach fifteen millimetres or more in length. As a 

 rule, also, the Palaeozoic Ostracoda are plain, or are simply striated 

 or granular; whereas the Mesozoic and Tertiary forms are com- 

 monly ornamented with projecting tubercles, or in various other 

 ways. As regards their general distribution in time, the Ostracoda 

 certainly commence in the Upper Cambrian, and are even doubt- 

 fully represented in the lower division of this formation. They 

 existed under many and varied types in the Ordovician and Silurian, 

 and are abundant in all the succeeding formations, till the Recent 

 period is reached. As matter of course, the remains of this group 

 of Crustaceans with which we have chiefly to do, are principally 

 those of the marme members of the order, and this is especially 



