210 



PALAEONTOLOGY 



or lobed. Includes two pelagic families (both Ord.- 

 Dev.) Lichadida, with very thin crust, and Acidaspidce, 

 very spiny. The rather isolated genus Bvonteus (Ord- 

 Dev.) shows some relations with these, but it is isopygous, 

 and its pygidium has an entire margin and the axis is 

 very short. 



ORDER II.: PROPARIA. 



Facial suture cutting lateral margin. The most 

 primitive genus is the Middle Cambrian Burlingia, which 

 is micropygous, but almost as broad as long. Asso- 

 ciated with it is the small isopygous Pagetia, which 



FIG. 60. CHEIRUROS BIMUCRONATUS (MURCHISON), WENLOCK 



LIMESTONE. 

 (Natural size.) (Original.) * 



retains the facial suture lost in the commoner forms 

 Microdiscus (Camb.) and Agnostus (Fig. 59, *, Camb. Ord.). 

 These three genera have the smallest numbers of free 

 somites of any trilobites 2, 4 and 2 respectively. 



The more familiar Proparia are only known from the 

 Ordovician onwards : they have nearly always eleven free 

 somites. 



The Encrinurida have very narrow free cheeks with an 

 almost straight facial suture, and small eyes, sometimes 

 on long stalks ; a long pygidium with many pleurae, and 

 often a still larger number of divisions of the axis. The 

 Cheirurida (Fig. 60) have a facial suture, with right-angled 



