3 o8 OUR COMMON BRITISH FOSSILS. 



ornamentation. Ammonites range in size from the 

 lovely A mmonites lautus of the Gault, not bigger than 

 a threepenny-bit, to the A. giganteus of the Upper 

 Oolite, as large as a cart-wheel. 



The Cephalopoda first make their appearance in 

 the Lower Tremadoc rocks. One form, Cyrtoceras 

 precox, found at Llanerch, west of Tremadoc, is the 

 oldest known. Orthoceras sericeum appears in the 

 Upper Tremadoc strata at St. David's, Llanwern, and 

 elsewhere. The Orthocemtites (as this genus is usually 



Fig. 306. Plan of foliation of edges of chamber of Ammonite. 



called when we speak of it in the plural) are most 

 numerous in the Upper Silurian beds, as at Woolhope, 

 Wenlock, and Ludlow. At the latter locality they 

 are very numerous, and some species attain a large 

 size. About thirty species of Orthoceras and Nautilus 

 are known from the Wenlock formation alone ; 

 twenty-four species of Orthoceras occur in the higher 

 Ludlow series. The Aymestry limestone is singularly 

 rich in places, in various kinds of Nautilidcz. Indeed, 

 whenever the Upper Silurian strata are fossiliferous, 



