FOSSIL CORALLINES. 37 



have been left. The most probable zoological position 

 of the Oldhamia is among the Hydrozoa, of which the 

 " sea-firs " (commonly mistaken for seaweeds) which 

 may be seen abundantly clustering on the backs of 

 oyster-shells in any fishmonger's shop, are the most 

 familiar examples. The Graptolites, which are so 

 numerous in the Lower Silurian rocks, as we shall 

 presently see, probably belonged to another division 

 of the same class of lowly organized marine animals. 



Two species of Oldhamia are known to geologists, 

 each distinctly marked from the other. Both are 

 found in the same locality viz. the Cambrian rocks 

 of Bray Head, about four or five miles from Dublin. 

 The place is easily reached, and will not soon be for- 

 gotten by the geological student. The rocks where 

 the Oldhamia occur are beyond the village, and form 

 the southern horn of the bay. They are very smooth 

 and fissile, and almost of a claret-colour. The fossils, 

 which sometimes occur along with the borings and 

 traces of marine worms, lie in zones, for certain strata 

 yield them more abundantly than others. In the 

 neighbourhood of the bathing-place, where the sea- 

 water appears unusually pure and green in com- 

 parison with the claret hue of the rocks, the Oldhamia 

 may be gathered in abundance. The species antiqua 

 also occurs in yellowish shales of the same geological 

 age, in Carrick mountain, county Wexford. Hitherto, 

 both species have been limited to the Cambrian rocks 

 of Ireland, where, however, they do not seem to have 



