268 



ME. T. ROBEETS ON THE UPPEE JURASSIC ROCKS 



The external form of the two figured species is not unlike that of 

 some Nerincm ; but on examining the inside of the whorls and the 

 columella, the spiral thickenings or folds, so characteristic of the 

 genus Nerincea, are not seen, nor is there any thing to indicate them 

 in either specimen except the " two broad faint grooves seen in 

 places upon the inside cast," mentioned by Mr. Keeping (op. cit. 

 p. 94) as occurring in N. tumida. These faint grooves are un- 

 doubtedly due to a slight variation in the thickness of the shell, but 

 are totally different from the spiral thickenings of Nerincea. 



The specimens have been submitted to Mr. Hudleston, and he is of 

 opinion that Nerincea, sp. (op. cit. fig. 7), is an undoubted Cerithium; 

 and that N. tumida, Keeping, belongs to the Cerithiadae rather than 

 to the j^erinoeidae. 



The other specimens, being derivatives, may have come from the 

 underlying Corallian.] 



Discussion. 



Mr. Etheridge said it was difficult to criticize this paper until 

 it appeared in print. It dealt with a mass of detail, and Mr. 

 Roberts had had peculiarly favourable opportunities for working out 

 the question both in Switzerland and in Cambridge. The work must 

 be compared with that done by Mr. Hudleston and Mr. Blake. The 

 speaker thought that the Pterocerian was represented at Portland ; 

 but the Portland and Kimeridge beds of England were peculiar, 

 and could not be exactly correlated abroad. 



Mr. Hudleston agreed with Mr. Etheridge as to the advantages 

 under which Mr. Roberts had studied the Jurassic rocks. Mr. 

 Roberts had lately undertaken a very difficult task, the examination 

 of the Jurassic beds near Cambridge, and had now entered on an 

 even more difficult inquiry. It was not very easy to find a classifi- 

 cation that would fit all countries. The Oxfordian and Callovian 

 were comparatively simple, on account of their fossils being widely 

 distributed, and, to some extent, the Corallian was not difficult to 

 trace, though there was a difficulty about the zone of Cidaris flori- 

 c/emma. The higher Oolites were more difficult to correlate. Thus 

 Nerincea is wanting above the Corallian in England (except a re- 

 ported occurrence in the Neocomian), but it is said to abound in 

 Kimeridgian and Portlandian on the continent. Undoubtedly our 

 Kimeridgian is abnormal. The 700 feet of Upper Kimeridge 

 in Kimeridge Bay would be classed as Portlandian on the conti- 

 nent, and Blake proposed to distinguish even lower beds as Bolonian. 

 The Ammonites afforded some clue to the relations of the beds. The 

 application of the term Portlandian to beds representing true Kim- 

 eridge was objectionable. He was surprised to hear Mr. Ethe- 

 ridge say that representatives of Pterocerian beds occurred at 

 Portland. So far as Mr. Blake and he had been able to ascertain, 

 no such representatives could be detected. 



Prof. Hughes said that the so-called Keocomian of Upware rested 

 on Coral Rag, and the fossils might have been derived. He called 



