THE TRILOBITA AND OTHER ARTHROPODA 217 



The Ostracoda are more thoroughly bivalve than 

 Estheria, as the two valves completely enclose the whole 

 body, and they are calcareous. They never show any 

 " growth-lines," and are nearly all of too small a size to 

 be lamellibranchs. The Silurian Leperditia is a giant 

 among ostracods, being nearly 20 mm. in length : a 

 glance at the hinge will show that it is not a lamelli- 

 branch : there is also a tubercle on each valve, antero- 

 dorsally, which could not occur in a molluscan shell. 

 Although the majority of ostracods are marine, it is in 

 freshwater deposits (such as the Purbeck Marls and 

 Wealden Shales) that they occur in such abundance as to 

 attract attention in spite of their small size. Ostracods 

 range from Cambrian to Recent. 



The Cirripedia are the only arthropods which are 

 fixed. They are all marine, and occur sparsely in forma 

 tions from the Ordovician onwards. They secrete a 

 shell of five or more pieces, which shows little analogy 

 with that of any other arthropod ; and in the works of 

 early palaeontologists such as Cuvier and the Sowerbys 

 they will be found figured among Mollusca. 



The Phyllocarida are an almost extinct group, mainly 

 Palaeozoic, differing from phyllopods in little else than 

 the great restriction in the number of somites, and the 

 lesser backward extension of the head-shield. Some 

 forms (Hymenocaris, Echinocaris) are fairly common fossils 

 from Cambrian to Devonian. 



The Isopoda (wood-lice) are terrestrial forms, with a 

 curious external resemblance to trilobites, which some 

 palaeontologists regard as a real relationship. Unlike 

 trilobites, however, their number of somites is fixed. 

 They are rare as fossils, though found occasionally from 

 the Devonian upwards : one species, Archceoniscus brodiei, 

 occurs in enormous numbers in certain of the Lower and 

 Middle Purbeck beds of the South of England. 



