254 



MINERALS AND GEOLOGY 



FIG. 167. 



Calymene Senaria in 

 " rolled up" con- 

 dition. 



enable the creature to sink with greater rapid- 

 ity into deeper water in moments of danger or 

 alarm. Fig. 167 is an example of a trilobite in 

 this " rolled up" condition. 



The pygidium, or shelly covering of the tail or 

 abdomen, consists of a single piece, arising probably 

 from consolidated segments. Very commonly, it 

 is found detached from other portions of the body. Its outline is 

 either rounded, with smooth or digitated margin, or is more or less 

 pointed ; and it sometimes terminates in a long spine, or in several 

 spinous processes. It shows very frequently a distinct axis, with 

 pleurae-like, lateral furrows, but is quite smooth or without furrows 

 in some species. In some genera, also, it is exceedingly small 

 ( Paradoxides, &c.), whilst in others ( Illcznus, Asaphus, &c.), it 

 equals the head-shield in size. 



The trilobites may be arranged broadly under four leading groups : 

 Pusilliformes, Latiformes, Frontones and Conifrontes. 



The Pusilliformes constitute a group of very small and somewhat 

 doubtful trilobites represented by the genus Agnostus. In this form 

 the thorax consists of only two segments, whilst the head-shield and 

 pygidium are quite large in comparison and of nearly equal size. 

 The type is essentially Cambrian, but lower Silurian examples are 

 also known. Several species have been obtained from the limestone 

 bands intercalated with the graptolitic states of the LeVis formation 

 (Quebec), but usually without the thorax. Figure 167 

 shows the head-shield and pygidium of Agnostus Cana- 

 densis from that formation. 



The group of Latiformes comprises a series of un- 

 doubted trilobites, mostly of considerable size and broad 

 form, with large head-shield and pygidium thus differ- 

 ing essentially from the more elongated many-ringed 

 forms of the typical Frontones and Conifrontes. The 

 principal genera of Canadian occurrence, comprise : Asaphus and 

 Illaenus ', but the group includes, also, Bathyurus, Bronteus, Lichas, 

 Dikelocephalus and other genera. Asaphus is distinguished by its 

 eight body-segments, its large head-shield with feebly-elevated and 

 (as a rule) unfurrowed glabella, its forked or horse-shoe-shaped 

 hypostoma, its large and broad pygidium, and other characters. The 



FIG. 167 bis 



Agnostus 



Canadensis. 



Hillings, 

 Levis For- 

 mation. 



