342 ARTHROPODA. 



dosed by an adductor muscle,, the insertion of which is marked 

 in the interior of each valve ly a tubercle, pit, or group of spots, 

 or ly loth spots and a pit. The branchiae are attached to the 

 posterior jaws, and there are only tivo or three pairs of feet, 

 which subserve locomotion, but are not adapted for swimming. 



Of the living Ostracode Crustaceans, a great many inhabit 

 fresh water (Cypris) ; others live in fresh or in brackish 

 waters (Candona) ; lastly, others are exclusively confined 

 to the sea (Cythere arid Cypridind). They generally swarm 

 in the localities in which they occur, and from their habit 

 of periodically shedding their valves, considerable accumu- 

 lations of their shells may be formed under favouring cir- 

 cumstances. 



It is only the carapace-valves of the Ostracode Crusta- 

 ceans 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 palaeon- 

 tologist 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 an- 

 other, or, lastly, the surf ace -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, appear to represent the eye ; and the 

 grooves and intervening lobes, which are found in many 

 forms, have been aptly compared by Barrande to the fur- 

 rows 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 cara- 

 pace are similar. The Mesozoic and Tertiary Ostracoda are 

 very small, and the same is true of a large number of 

 Palieozoic species; but among the latter we find some com- 



