650 STRATIGRAPHICAL GEOLOGY. — [Boox VI. 
rocks of the south-east of Ireland and is named Oldhamia (Fig. 320). 
For many years it was considered to be a sertularian zoophyte, 
subsequently it was referred to the calcareous alge; but its true 
grade seems still uncertain. 

Fia. 320.—Grovur or Lower CAMBRIAN FOssis. 
1, Arenicolites didymus (Salt.); 2, Oldhamia antiqua (Forbes); 3, Theca corrugata 
(Salt.); 4, Protocystites menevensis( Hicks) (2); 5, Protospongia fenestrata (Salt.) (and 
enlarged +); 6, Discina pileolus (Hicks) (and enlarged) ; 7, Obolella maculata (Hicks). 
organisms appear even here, as it were on the very threshold of authen- 
tic biological history, to have reached their full structural development. 
Some of them indeed were of dimensions scarcely ever afterwards 
equalled and already presented great variety of form. Individuals 
of the species Paradowides Davidis ave sometimes nearly two feet 
long. But with these giants were mingled other types of diminutive 
size. It is noteworthy also, as Dr. Hicks has pointed out, that while 
the trilobites had attained their maximum size at this early period, 

