262 TRILOBITES. 



antennae. The feet were membranous. "We know only one, 

 Calymene, from America, where these appendages are pre- 

 served. Many trilobites had the power of rolling themselves 

 into a ball, from which we conclude that the abdomen had no 

 special appendages at its extremity. 



The tegumentary skeleton of this order is formed in 

 general of two distinct layers : the external is thin, often 

 granulated, and ornamented ; the internal, which is always 

 present, is denser than the former. The Trilobites all belong 

 to the palaeozoic rocks. They lived in numerous families, 

 and presented an immense association of individuals, but 

 were much restricted as regards the number of the genera 

 and species. 



Burmeister distributes the Trilobites into six families. 



The ASAPHIDJE had the power of rolling the body into a 

 ball : their dorsal axis is not contracted posteriorly ; the 

 carapace is often sculptured witli lines, but is seldom granu- 

 lated. Most of the genera have less than ten rings to the 

 thorax. It comprises Illtenus, Bumaster, Arcliegonus, Dys- 

 planus, Amphyx, and Asaphus. These genera range between 

 the lower Silurian and the carboniferous stages. 



The CALTMENIDJE had the power of rolling themselves into 

 a ball ; the dorsal axis of the body is contracted posteriorly ; 

 the carapace is often granulated, and they had for the most 

 part more than ten rings in the thoracic division of the body. 

 It comprehends Calymene, Cypliaspis, fJiacops, ^Eonia, and 

 Homalonotus (fig. 176). This family ranged through the 

 Silurian, Devonian, and carboniferous periods. 



FIG. 176. Homalonotus delphinocephalus. Konig. 



The HAKPESIDJJ. The lateral lobes of the body-rings are 



