1864.] TATE CRETACEOUS KOCKS OF IRELAND. 33 



VI. Pal-tEontological Summary. 



In the Glauconitic Sands, which may be further designated the 

 " Zone of Exogyra conica,^^ 36 species have been observed ; of these 

 12 species, or 34 per cent., pass np into the higher zone, 7 of them 

 appearing in the Chloritic Sandstones. With but two or three ex- 

 ceptions, the species that rise upwards are of rare occurrence in the 

 superior zones into which they extend, and similarly those that pass 

 down into the Glauconitic Sands from higher zones are rare in them. 



In the " Grey Marls and YeUow Sandstones," which may further 

 be called the " Zone of Ostrea carinata," 19 species have been met 

 with, 7 are confined to it, 10 occur also in the Glauconitic Sands 

 below, and 8 are common to it and the Chloritic Sandstones. 



In the Chloritic Sandstones and Sands, which embrace two zones 

 — the Zone of Inoceramus Crispi and that of Exogyra columba, — 78 

 species have been discovered, 59 of which, or 78-2 per cent., are 

 peculiar to them ; 10 occur in the lower zones, and 9 extend into 

 the White Limestone : the majority of these are Brachiopods ; the 

 only anomalous species is, perhaps, Spondylus sjpinosus, which, how- 

 ever, is rare in the White Limestone, though exceedingly abundant 

 in the Zone of Inoceramus Crispi (2). 



The species of the Zone of Exogyra columba are, with some five or 

 six exceptions, distinct from those of the Zone of Inoceramus Crispi. 



In the Chloritic Chalk 24 species and varieties have been found. 

 The majority of the Spongiadce are peculiar to it, BelemniteUa mucro- 

 nata, Turritella uniccwinata, and the Echinoclermata appearing also 

 in the White Limestone. 



The White Limestone has yielded 83 species, 68 of which are 

 peculiar. 



These figures show that each palseontological zone has a distinctive 

 character ; that the Zones of Exogyra conica, Ostrea carinata, Ino- 

 ceramus Crispi, and Exogyra columba are more closely related one to 

 the other than they are severally to the zones of the Upper Chalk ; 

 that, of these, the fauna of the Zone of Exogyra conica presents a 

 striking contrast to that of the Zones of Inoceramus Crispi and Exo~ 

 gyra columba, the hiatus being bridged over by the species in the 

 intermediate zone, which, however, are more closely related to those 

 of the Zone of Exogyra conica. 



So, again, the fossils of the Upper Chalk present marked points of 

 contrast to those of the Hibernian Greensand ; and its subdivisions, as 

 employed, are good horizons both as regards their paleeontological and 

 their lithological characters. 



VII. Conclusions. 



1. The Cretaceous rocks of. Ireland are referable to two forma- 

 tions — the Hibernian Greensand and the Upper Chalk. 



2. The Hibernian Greensand is divisible into well marked Utho- 

 logical and palseontological zones, and is the equivalent in miniature 

 of the Etage Cenomanien of D'Orbigny. 



3. The Zone of Exogyra conica represents the basement-beds of 

 VOL. XXI. — part I. D 



