TRILOBITA. 



521 



"With regard to the condition of the under surface of the Trilo- 

 bites, the progress of our knowledge has been slow, and is still in 

 some respects far from complete. Specimens showing the inferior 

 surface are exceedingly rare, and until of late years the only struc- 

 ture which had been detected on this aspect of the animal was the 



Fig. 



575. — Glabella and pygidium of Dikellocephalus magnificus, Quebec group (Lower 

 Ordovician). (After Billings.) 



upper lip. The margin of the head-shield (as that of the pygidium 

 also) is turned under in the form of a broader or narrower down- 

 ward and inward inflection or "doublure" (fig. 377, d\ and to the 

 centre of this is attached the lip-plate or " hypostome." The form 

 of this varies, but usually it is either an oval gibbous plate or it is 

 broad and deeply forked behind (figs. 376, 

 377, h). Beside this lip-plate, in a speci- 

 men of Asaphus platycephalus, Dr Henry 

 Woodward detected a jointed filament (fig. 

 376,/), apparently attached to a maxillary 

 plate, and representing a " maxillary pal- 

 pus." At a later period, Mr Billings de- 

 scribed a specimen of Asaphus platycepha- 

 lus, showing the under surface of the body, 

 with a pair of apparently jointed appen- 

 dages attached to each segment of the 

 thorax ; but the nature of these structures 

 as ambulatory legs was not universally admitted. The correctness 

 of the view held by Mr Billings as to the nature of the thoracic 

 limbs in Asaphus platycephalus has, however, been completely 

 established by a specimen of Asaphus megistos, from the Ordovi- 

 cian rocks of Ohio, which shows the under surface with its appen- 



Fig. 376. — Buccal organs of 

 Asaphus platycephalus (after 

 Woodward), h, Hvpostome ; /, 

 Palpus; m, Maxilla (?). 



