Tremadoc Slates. 65 



the Cambrian strata, while G. variolaris is common 

 to the Lingula and Menevian strata. Paradoxides 

 Harknessii, P. Hicksii, and P. aurora are common 

 to all three formations, and P. Davidis to the two 

 higher divisions. The same kind of passage of species 

 upward from the Cambrian slates and grits to the 

 Lingula flags, may be observed in some of the few 

 genera and species of Brachiopoda, Lamellibranchiata, 

 and Pteropoda, and I have specially insisted on this, in 

 connection with the fact, that at the junction of the 

 so-called three formations, there is no marked line of 

 division, but a lithological gradation from the lower 

 to the higher strata, accompanied by the passage of 

 species from lower to higher geological horizons. 



The Tremadoc slates succeed the Lingula flags, and 

 may be considered as an upper member of that series, 

 while the red and grey Cambrian rocks form a lower 

 member. There is no clear lithological boundary-line 

 between them, and the whole lie conformably. Four- 

 teen genera of Trilobites are known in Wales from these 

 strata, the most characteristic of which are Asaphus 

 Homphrayi, &c., Angelina Sedgwicki, Psilocephalus 

 inflatus, &c., and Niobe Homphrayi. Several of the 

 genera of Trilobites are common to the Lingula flags 

 and the Cambrian beds below. Of these, Agnostus prin 

 ceps is found in the Menevian beds, and Conocoryphe 

 depressa, C. invita, and Olenus alatus in the Lingula 

 flags. Orthis Carausii is a characteristic brachiopod, 

 and Lingulella Davisii and L. lepis are common to 

 these strata and to the Lingula flags, together with 

 Obolella plicata and 0. Salteri, while 0. sagittalis 

 ranges from the Cambrian up to the Tremadoc slates. 

 In Eamsey Island near St. David's many Lamelli- 

 branchiate molluscs have been found by Mr. Hicks, of 



