206 PALAEONTOLOGY 



swimming life, and became adapted to those special con- 

 ditions. Adaptation to life in the mud is shown by a 

 shovel-like head ; the eyes as far from the margin as 

 possible (to be out of the mud, where they are useless) 

 and sometimes lifted up on long stalks, or else the eyes 

 are lost altogether ; the pygidium sometimes ends in 

 a spine. Adaptation to a pelagic life is shown by a more 

 or less globular head with eyes near the margin, some- 

 times by very thin shell, peculiarly shaped pygidium, and 

 great development of spines (as a protection). 



There remain the static or differential characters, the 

 chief of which is the course of the facial suture. Others 

 of less value, because liable to be modified by progression 

 or adaptation, are the form and proportions of glabella 

 and cheeks, and of axis and pleura. 



The recognition of the importance of the facial suture 

 is due to the late Prof. Beecher of Yale, who in 1895 

 proposed the names Opisthoparia and Propavia for the 

 orders with the two types of suture. Unfortunately a 

 few trilobites, mostly blind, show neither type, and 

 in these Beecher thought the facial suture was marginal 

 or sub-marginal : he therefore united them as Hypoparia. 

 More recent discoveries seem to show that these forms 

 had lost their sutures through their blindness, and can be 

 traced back partly .to opistho- and partly to pro-parian 

 ancestors. Only two orders appear necessary therefore. 

 Of these, Opisthoparia is much the larger, and is divided 

 by Prof. Swinnerton of Nottingham into sub-orders as 

 shown below. 



