209 



suture cuts the genal angle. It includes two important 

 genera (both Ord.-Dev.) the heteropygous Calytnene, 

 already described, and the isopygous Homalonotus, with 

 several sub-genera, of which Trimerus (Fig. 59, /), with 

 triangular head and pygidium, is the best known. 



SUB-ORDER 3. Trinucleida. A somewhat divergent 

 series, of which the more generalized ancestral forms 

 are not known. The Havpedidce (Ord.-Dev.) are micro- 

 pygous, with many thoracic somites, but specialized in 

 having a brim like that of Trinucleus, except that it 

 extends far back beyond the genal angles. Ampyx re- 

 sembles Trinucleus in thorax and pygidium, but has no 

 head-brim, has very narrow free cheeks, and a long 

 median spine in front. Trinucleus (Ord.) has been de- 

 scribed and its eye-bearing ancestor Orometopus (Upper 

 Camb.) referred to already. Possibly to be placed here 

 are two small isopygous forms Shumardia (Upper 

 Camb.) and /Eglina (Ord.), the latter a pelagic form with 

 enormous eyes (Fig. 59, h), probably a nocturnal animal 

 descending to the depths of the sea by day and feeding at 

 the surface at night. 



SUB-ORDER 4. Odontopleurida. Glabella sub- 

 divided into lobes bearing little relation to the original 

 segmentation ; free cheeks wide; micropygous, pygidium 

 with pleura projecting so as to make the margin toothed 



FIG. 59. TRILOBITES. 



a, Olenellus thompsoni (Halt), Lower Cambrian. Parker's Quarry, 

 Georgia, Vermont. (x.) (After Walcott.) b, Ogygiocaris \_Ogy gia\ 

 buchi (Brongniart), Llandeilian. (Xj. ) (After Salter.) c, Olenus 

 cataractes Salter, Upper Cambrian. (Xj.) (After Salter. ) d, Illcenus 

 davisii Salter, Ordovician. (x.) (After Salter.) e, Bumaslus 

 barriensis (Murchison), Silurian. Side view of an enrolled specimen. 

 (Xj.) (After Salter.) /, Trimerus [ Homalonotus] delphinocephalus 

 Green, Silurian. (X^.) g, Phillipsia derbiensis (Martin), Lower 

 Carboniferous. (x3.) (After H. Woodward.) h, sEglina binodosa 

 Salter, Ordovician. (Natural size.) (After Salter.) i, Agnostus 

 princeps Salter, Upper Cambrian . (X?.') (After Salter.) 



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