500 PLIOCENE MOLLUSCA. 



Nucula tenuis. Tapes aureus (local). 



Leda ohlongoides. „ virgineus. 



Gardium edule, tliin variety. Gorhula striata. 



„ Serriprs groinlandiciim. Scrohicularia pij^erata (local). 



Astarte comprcssa. Mactra. suldruvcata. 



„ {Tridonfa) horpulis. ,, elliptica. 



Tellina (Macoma) lata. Mija arenaria. 



„ „ obliqua. Together with a number of non -marine 



forms. 



The arctic species Astarte [Tridontn) horealis is specially characteristic of the 

 northern part of the Icenian Crag, occurring frequently near Norwich and at the 

 Weybourne horizon, but it is less so at the southern localities. It may be noticed 

 that while the sub-estuarine Mi/a aremaria is common in the Icenian, it is almost 

 unknown from the Coralline and Waltonian horizons, where the more distinctly 

 marine Mija, fruvcata is rather aliundant. For the guidance of collectors when 

 dealing with imperfect specimens it may l)e remarked that the hinge of these two 

 species is different. (See figures in a paper by Sir H. H. Ho worth, Proc. Zool 

 Soc, !>. 755, figs. 241—2-13, 1901».) 



Chillesfoed Horizon. 



Overlying in places the Red, NorAvich, and in one locality the Coralline Crag, 

 are some beds of dark laminated clay,^ always highly micaceous, which maintain 

 the same character from Chillesford in Suffolk to Burgh in Norfolk,^ where, in my 

 opinion, they abruptly disappear. When plotted on a ma}) they are found to 

 arrange themselves along a narrow and sinuous belt, as if representing the bed of 

 a former and muddy estuary. Differing essentiall}^ from the coarse sandy matrix 

 of the typical Crag, they must have originated under different conditions. When 

 examined microscopically the late CI. Reid could not find this deposit to contain a 

 single Crag fragment of sand, the grains consisting of (piartz, small and little 

 worn. The view that the Chillesford Clay is of an estuarine character seems to be 

 supported by the fact that at one spot the complete and undisturbed skeleton of a 

 large cetacean, which Dr. Crisp ascertained to be 31 feet in length, was found in it.'^ 



At the Church-pit at Chillesford typical Chillesford Clay is underlain conform- 

 a})ly by a bed of stratified sand containing a special group of fossils, which are not 

 the drifted shells of dead animals like those of the Red and Norwich Crags, but 

 for the most part double and thin and fragile. They are of a northern type but 

 never contain the characteristic form of the Weybourne horizon, Tellina. (Macoma) 

 baltJdca. In the superabundant mica they contain, the Chillesford beds resemble 



1 First described by Prestwich in 1849 (Quart. Journ. Geol. Soc, vol. v, p. 345). 

 ^ Eep. Brit. Assoc. (Norwich), Trans., p. 61, 1868. 



