TRILOB1TES AND OTHER FOSSIL CRUSTACEA. 185 



animals are not an exception to the general palaeon- 

 tological rule that the simpler always precede the 

 more complex species of the same genus or class. 

 Nevertheless, this simple and elementary Trilobite is 

 found in company with a more highly developed kind. 

 Agnostus is usually found in large shoals, something 

 after the manner in which the carapaces of the ancient 

 water-fleas are met with in some of the Coal-measure 

 shales. Owen suggests that this disposition of Agnos- 

 tus is "as if it were the larval form of some large 

 Trilobite." The young of all Crustacea usually asso- 

 ciate together in shoals, and this suggestion might 

 therefore be reasonably taken in consideration with 

 what has already been said on the subject. 



The compound eyes of Trilobites are usually thickly 

 placed on raised half-moon-shaped ridges, and the fact 

 that the sockets are so well preserved speaks plainly 

 of the quiet way in which the fine mud was deposited 

 in which the animals were buried and ultimately fossi- 

 lized. Dr. Buckland spoke of these ridges as being 

 " like a circular bastion, ranging nearly round three- 

 fourths of a circle, each commanding so much of the 

 horizon that where the distinct vision of one eye 

 ceased, that of the other began." He also very saga- 

 ciously referred to the form of the ridges and their 

 position on the head-shield as "peculiarly adapted to 

 the uses of an animal destined to live at the bottom 

 of the water : to look downwards was as much im- 

 possible as it was unnecessary for a creature living at 



