156 ANNULOSA. 



be left out of consideration, as they are not certainly known to 

 occur in the fossil condition. There are also good reasons for 

 the belief that the Trilobites should be placed amongst the 

 Malacostracous Crustaceans, in or near the order Isopoda. In 

 the absence, however, of unassailable evidence, the Trilobites 

 may be safely retained in the vicinity of the Phyllopods, to 

 which they show undoubted affinities. 



ORDER OSTRACODA. 



Minute Crustaceans having the entire body enclosed in a shell or 

 carapace, which is composed of two valves united along the back by 

 a membrane. The valves are capable of being closed by an adduc- 

 tor muscle, the insertion of which is marked in the interior of each 

 valve by a tubercle, pit, or group of spots, or by both spots and a 

 pit. The branchice are attached to the posterior jaws, and there 

 are only two or three pairs of feet, which subserve locomotion, but 

 are not adapted for swimming. 



Of the living Ostracode Crustaceans, a great many inhabit 

 fresh waters ( Cypris] ; others live in fresh or in brackish waters 

 ( Candona) ; lastly, others are exclusively confined to the sea 

 ( Cythere and Cypridina). They generally swarm in the locali- 

 ties in which they occur, and from their habit of periodically 

 shedding their valves, considerable accumulations of their shells 

 may be formed under favouring circumstances. 



It is only the carapace-valves of the Ostracode Crustaceans 

 that are preserved in the fossil condition; 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 an- 

 other, or, lastly, the surface-ornamentation. Partly for this 

 reason, and partly because the number of known fossil Ostra- 

 coda is very large, it will not be advisable here to do more than 

 give an outline of the general distribution of the order in time. 



The Ostracode Crustaceans appear to have been amongst 

 the earliest representatives of their class, abounding as they do 

 in many Lower Silurian deposits. Amongst the more import- 

 ant genera which are represented in the Silurian Rocks may 

 be mentioned Primitia (fig. 100, a), Leper ditia, Beyrichia, Ento- 

 mis, Cypridina, Cytherella, Cythere, and Bairdia. Of these the 

 genus Primitia is exclusively confined to the Silurian rocks, 

 whilst the great genus Leperditia arrived here at its greatest 

 development. On the other hand, the genera Cytherella, 



