i6o 



ANNULOSA. 



again, the carapace appears to be bent, but not divided or 

 hinged, along the dorsal line, and its shape is pod-like (fig. 

 102, a), with an abrupt posterior truncation. The surface of 

 the carapace is marked with " fine, obliquely longitudinal, im- 

 bricated striae " (M'Coy). 



Fig. 102. Palaeozoic Phyllopods : a Ceratiocaris papilio, Upper Silurian (Salter). 

 b Hyntenocaris vermicauda, Upper Cambrian (Salter). c Discinocaris Browniana, 

 Lower Silurian (Original), d Peltocaris aptychoides, Lower Silurian (Woodward). 



ORDER TRILOBITA. 



Crustaceans in which the body is usually more or less distinctly 

 trilobed; there is a cephalic shield, usually bearing a pair of sessile 

 compound eyes ; the thoracic somites are movable upon one another, 

 and are very variable in number ; the abdominal segments are 

 coalescent, and form a caudal shield ; there is a well-developed 

 upper lip or " hypostome" 



As regards the general structure of the Trilobites, the body 

 was protected by a well-developed chitinous shell or " crust," 

 which covered the whole dorsal surface of the body, and on 

 which no appendages have ever been discovered except the 

 eyes. The under surface of the body must, in many genera at 



