TRILOBITA. 



357 



rounded, with an " entire " margin ; but it may be prolonged 

 into a shorter or longer spine or "mucro" (fig. 207), and 

 the ends of the pleurse may also be extended into spine-like 

 projections (fig. 213). The number of rings in the tail varies 

 from two (Sao hirsuta) to twenty-eight (species of Amphion). 

 With regard to the condition of the under surface of the 

 Trilobites, the progress of our knowledge has been slow, and 

 is still far from complete. They appear to have lived on 

 muddy bottoms, in shallow water, and they probably swam 

 on their backs, as do the modern Apus and the larvae of 

 Limulus ; but in spite of the innumerable specimens which 



Fig. 213. Glabella and pygidium of Dikellocephalus magnificus, Quebec Group 

 (Lower Silurian). (After Billings.) 



have been exhumed from the rocks, the only structure which 

 had, until within the last few years, been detected on the 

 lower surface was the upper lip. The margin of the head- 

 shield (as that of the pygidium also) is turned under in the 

 form of a downward and inward inflection, or "doublure;" 

 and to the centre of this, in many forms, is attached the 

 "lip-plate." The shape of this varies, but it was usually 

 a broad forked plate situated in front of the mouth, and 

 doubtless corresponding with the upper lip " labrum" or 

 " hypostome " of living Crustaceans, its form closely re- 

 sembling that of the lip-plate of the recent Apus, one of 

 the Phyllopods. The next advance in our knowledge was 



