PHYLLOPODA ISOPODA. 263 



not extended horizontally in their whole length, but turned 

 downwards from the centre, and do not terminate in a 

 point, but with an- arched and rounded extremity, fur- 

 rowed on the surface along their whole length. The genera 

 Gmpocephalus, Ellipsocepliahis, and Harpes belong to this 

 ~ytribe ; they are confined to the upper Silurian and Devonian 

 stages. 



The OLENnxsj were incapable of rolling themselves into 

 a ball ; the caudal shield is very small, the axis of which is 

 many-jointed, but always shorter than the body : it includes 

 the genera Paradoocides and Olemis, which are limited to the 

 upper and lower Silurian stages. 



The ODONTOPLEUEIDJG have the form of the Ogygidce : 

 like them they w r ere incapable of assuming the globular 

 form. The abdominal shield is smaller than the thoracic, 

 and consists of few segments. It includes Odontopleura, 

 Arges, and Brontes of the upper Silurian and Devonian 

 stages. 



The OGYGID.E were incapable of rolling themselves into 

 a ball ; the lateral lobes of the rings of the body are situated 

 on the same plane, and do not curve or bend downwards, but 

 terminate posteriorly in a point or spine more or less pro- 

 minent, sometimes very long, which forms an obtuse angle 

 with the direction of the lobes. The abdominal shield is 

 simple, almost as large as the cephalic, and as long as the 

 thoracic, and is composed of many separate elements. In 

 this family is grouped Trinucleus, ' and Ogygia, which are 

 found in the Silurian stages. 



The EuuYPTEBiD-E, it is supposed, had no shell. "We only 

 know the existence of the family by the impression of 

 the body and head, with antennae, and compound eyes of 

 Eurypteriis, in the inferior stage of the Silurian rocks of 

 North America. 



Order PHYLLOPODA. Are naked or covered with a 

 bivalve carapace. Their body is divided into numerous 

 segments, which support foliaceous branchiferous feet. 

 Belonging to this order is Nebalia and Apus, from the car- 

 boniferous stage. 



Order ISOPODA, of which the common wood-louse, Onis- 

 cus, is typical. Have the thorax composed of seven segments 



