PHYLLOCARIDA. 



513 



Crustaceans, which are exclusively marine in habit. The recent 

 forms constitute a special division of the order {Xeb alia dee), dis- 

 tinguished, among other characters, by the want of a telson ; and 

 they are not known to be directly represented in the fossil state. 

 On the other hand, there are numerous fossil Crustaceans, chiefly 

 of Palaeozoic age, which appear to be properly referable to the 

 Phyllocarida, and which may be regarded as constituting a special 

 division of this order (Ceratiocaridce), distinguished more particu- 

 larly from the Nebaliada by the presence of a well-developed telson. 

 The earliest forms of this peculiar group of " Pod-shrimps," as they 

 have been termed, appear in the Cambrian period, while the group 



Fig. 365. — Palaeozoic Phyllocarida. a, Ceratiocaris papilio— Silurian (Salter) ; b, Hyuienocaris 

 7-ermicauda — Upper Cambrian (Salter) ; c, Discinocaris Broivniana, without the " rostrum " : — 

 Ordovician (Original); d, Pcltocaris aptychoides — Ordovician (Woodward). 



attains its maximum development in the Silurian, and underwent 

 almost total extinction subsequent to the close of the Carboniferous 

 period. The number of genera included in the family of the 

 Ceratiocaridce is very large, and many of these are only imperfectly 

 understood. It will therefore be sufficient here to allude briefly 

 to some of the more important and better known types. 



As the type of this group may be taken the genus Ceratiocaris 

 itself (fig. 365, a) in which the anterior portion of the body is en- 

 closed in an elongated or pod-like carapace, composed of two oval 



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