34 The Irish Naturalist. 



and not a single species, has been able to struggle on unaltered 

 through the long series of changes which Co. Dublin has 

 since then undergone, 



Thus at the present daj- the British seas contain one species 

 of brachiopod to every twenty-five species of lamellibranchs, 

 and every thirt3"-six of shell-bearing gastropods ; while in the 

 British Bala series we have the lamellibranchs and gastropods 

 about equal in point of species, and the species of brachiopods 

 equal to those of the two groups put together. Numerical 

 predominance of individuals is, however, a safer test of 

 relative importance ; and here at Portraine brachiopod life is 

 at once seen to be abundant, while true moUuscan individuals 

 are ver}^ rare. 



In the spring of 1891, Mr. Walcott recorded the buckler-like 

 plates of very early types of fishes, as occurring in Colorado, 

 in the Trenton series, the upper Ordovician of America.' So 

 the sea of Co. Dublin ma}- not have been absolutel}' destitute 

 of fish, but these remains are the oldest hitherto recorded, and 

 the first British fishes are found at the top of the Silurian. 



The Ordovician shales may be well seen in the southern 

 parts of the county, where the Dodder and other streams have 

 cut down sufficiently through the covering of drift. A fine 

 section occurs near Dargle Bridge on the road from Bray to 

 Knniskerry, and the schist formed from the shales by contact 

 with the granite is nowhere better seen than on the Killiney 

 shore. 



Fig' 5- 



Microscopic section of volcanic ash, north end of promontor}- of Por- 

 traine. Fragments of formerly glassy lavas, with felspar crystals, lie in 

 a compact gronnd of finer ash. x 30, 



On Ireland's Eye, moreover, the smoother ground on the 

 back of the quartzites (fig. i in our last number) is formed of 

 Ordovician shales. 



Nature, xliii, p. 425. 



