THE TRILOBITA AND OTHER ARTHROPODA 20$ 



suture cuts the genal angle. It includes two important 

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

 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 Harpedidce (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. Trinucleiis (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 Skumardia (Upper 

 Camb.) and Mglina (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 (Hall), Lower Cambrian. Parker's Quarry, 

 Georgia, Vermont. (Xj.) (After Walcott.) b t Ogygiocaris [Ogygia] 

 buchi (Brongniart), Llandeilian. (X^. ) (After Salter.) c, Olenus 

 cataractesSa\tev, Upper Cambrian. (Xj.) (After Salter.) d, Illcenus 

 davisii Salter, Ordovician. (X^.) (After Salter.) e, Bumastus 

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

 (X|.) (After Salter.) /, Trimerus {Homalonotus} delphinocephalus 

 Green, Silurian. (X|.) g, Pfiillipsia derbiensis (Martin), Lower 

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

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

 princeps Salter, Upper Cambrian . (Xil.) (After Salter. ) 



