no 



HISTORICAL PALAEONTOLOGY. 



of new forms some of them, like the great Asaphus tyrannus 

 of the Upper Llandeilo (fig. 47, a), attaining a length of a foot 

 or more, and thus hardly yielding in the matter of size to their 

 ancient rivals. Almost every subdivision of the Lower Silurian 

 series has its own special and characteristic species of Trilo- 

 bites; and the study of these is therefore of great importance 

 to the geologist. A few widly-dispersed and characteristic 



Fig. 50. Lower Silurian Brachiopods. a, and a' Or this biforata, Llandeilo-Cara- 

 doc, Britain and America ; ft, Orthis flabellulum, Caradoc, Britain ; c, Orthis sub- 

 guadrata, Cincinnati Group, America ; c' Interior of the dorsal valve of the same ; d, 

 Strophomena deltoidea, Llandeilo- Caradoc, Britain and America. (After Meek, Hall, 

 andSalter.) 



species have been here figured (fig. 47) ; and the following 

 may be considered as the principal Lower Silurian genera 

 Asaphus, Ogygia, Cheirurus, Ampyx, Calymenc, Trinucleus, 

 Lie has, Illcenus, ^Eglina, Harpes, Rcmopleurides, Phacops, 

 Acidaspis, and Homalonotus, a few of them passing upwards 

 under new forms into the Upper Silurian. 



Coming next to the Mollusca, we find the group of the Sea- 

 mosses and Sea-mats {Polyzoa) represented now by quite a 

 number of forms. Amongst these are examples of the true 

 Lace-corals (Retepora and Fenestella), with their netted fan-like 

 or funnel-shaped fronds; and along with these are numerous 

 delicate encrusting forms, which grew parasitically attached to 

 shells and corals (Hippothoa, Alecto, &c.) ; but perhaps the 

 most characteristic forms belong to the genus Ptilodictya (figs. 

 48 and 49). In this group the frond is flattened, with thin 



