PRESTWICH FOSSILIFEROUS IRONSANDS, N. DOWNS. 329 



From the greater prevalence of some of the more abundant Coral- 

 line Crag forms, I should feel inclined to place these beds in that group 

 rather than in the Red Crag. Thus the Scalaria subulata, Astarte 

 pygmcea, and Mactra triangulata, as also the genera Pyrula and Avi- 

 cula, are peculiar to the Coralline Crag ; the Terebratula grandis, As- 

 tarte Omalii, and Tellina donacina are also more particularly abundant 

 in this portion of the Crag, the former shell attaining a larger size than 

 in the Red Crag ; and of the genera Lucina, Kellia, and Lepton 

 there are a considerably greater number of species in the Coralline 

 than in the Red Crag. On the other hand, the Leda my alls and 

 Astarte compressa are said to occur only in the latter division ; 

 but the former is a very doubtful specimen. The remaining species 

 and genera are of about equal value with regard to either division. 

 Still the number of species at all recognizable are so few, that the 

 palseontological argument must be received with reserve. 



There are, however, some other points which tend to confirm in 

 some measure this synchronism, — points of physical structure and 

 geographical distribution. Thus, with regard to the latter point, 

 one main feature of the Coralline Crag contrasting with the Red 

 Crag is, that the former contains a considerably larger proportion of 

 species of southern, and the latter of northern affinities*. So in these 

 Lenham beds, the geographical distribution of the 1 1 recent and 5 

 extinct species in Mr. Wood's list (p. 334) shows a certain prepon- 

 derance of southern forms. The following Table exhibits this dis- 

 tribution : — 



CS o 



g 



'2 . 



O.S5 



PS^S 



V) 



u 



■wca 

 PS 



1. Dentalium costatum .. 



2. Emarginula reticulata 



3. Nassa prismatica 



4. Scalaria subulata 



5. Arcalactea 



6. Astarte digitaria. 



7 



8. 



9. 

 10. 

 11. 

 12. 

 13. 

 14. 



compressa 

 Omalii ... 

 pygmaea. 



Cytherea rudis 



Leda myalis 



Modiola modiolus .. 

 Mactra triangulata . . 

 Nucula nucleus 



15. Tellina donacina 



16. Terebratula grandis 



*? 



*? 



14 



12 



8 



6 



* See Mr. S. Wood's Monograph, published by the Palaeontographical Society, 

 for much valuable information on this subject. 



