534 



CRUSTACEA. 



shield may be rounded or spinose, and the glabella is not marked off 

 by conspicuous axal furrows. The facial sutures are discontinuous, 

 the eyes crescentic, the hypostome deeply forked, and the pygidium 

 may or may not show a conspicuous axis, its hinder extremity being 

 usually rounded, and its margin always entire. This important 

 genus is characteristic of the Ordovician rocks, and the species 

 have a world-wide distribution, some of the forms attaining the 

 extraordinary size of two feet in length. The genus Asaphus has 

 been divided into a number of sub-genera, of which Isotelus and 

 Megalaspis are the most important. In the former of these (fig. 

 392) the axis is very wide, the glabella is imperfectly marked off, 



Fig. 2,90.— Asaphus tyrannus. Ordo- 

 vician. (After Salter.) 



Fig. ^gi. — Ogygia Buchii. Ordovician. 

 (After Salter.) 



and the pleurae have rounded ends. In Megalaspis (fig. 393), on 

 the other hand, the axis is narrow, the glabella is very short, and 

 the head-shield is usually greatly extended in front. 



The genus Ogygia is closely allied to Asaphus in general form and 

 proportions ; but the axis of the pygidium is more conspicuously 

 marked than in most Asaphi (fig. 391), the hypostome is rounded 

 and not cleft, the glabella is distinctly furrowed, and the pleurae of 

 the thoracic segments have only rudimentary "fulcra." The species 

 of Ogygia are confined to the Ordovician rocks. This is also the 

 case with the genus Niobe, which is in some respects intermediate 

 between Asaphus and Ogygia, having the round hypostome and 

 lobed glabella of the latter, while it approaches the former in its 

 wide glabella and its obtuse and faceted pleurae. The Ordovician 



