90 Mr. W. H. Benson on the Shell and Animal of Hybocystis. 



deposition of an immediately consecutive formation, or after a 

 longer or shorter interval during which other formations might 

 be deposited, the same marine population might reappear in the 

 same locality and give rise to identical organic debris, enclosed 

 in superior strata. Thus arc formed what are termed colonies in 

 Geology. It is probable, however, that this phenomenon could 

 only present itself when the same species had continued to live 

 in the interval, perhaps exceedingly reduced in number, in some 

 other locality. We have nevertheless shown how it may happen 

 that remains of perfectly identical species may pass into rocks 

 of a nature quite different, and deposited by very different 

 seas. 



7. There probably exist no formations immediately superposed 

 upon each other, no consecutive faunas and floras, without cer- 

 tain organisms being common to both. The number of common 

 species may vary between 0*01 and OIO. 



8. When, however, in certain localities there have been sudden 

 movements of the soil, heating of the crust of the earth, emissions 

 of sulphurous vapours, carbonic acid, or other injurious gases, 

 long interruptions in the formation of deposits, upheavals of 

 strata, &c, it most frequently happens that the passage of species 

 from one stratum to another is more rare than when the deposits 

 have been formed regularly and without any interruption. 



9. The average absolute duration of organisms was sufficiently 

 long to give us no reason for astonishment at the important 

 differences presented by species in this respect, although the 

 history of these species is often told us only by strata of but 

 slight thickness, so that it often happens that we regard as 

 simultaneous, phenomena which have been separated by long 

 periods of time. 



[To be continued,] 



VIII. — Observations on the Shell and Animal of Hybocystis, a 

 new genus o/Cyclostomidse, based ora Megalomastoma gravidum 

 and Otopoma Blennus, B. ; with Notes on other living Shells 

 from India and Burmah. By W. H. Benson, Esq. 



Dr. Pfeipfer has divided the genus Megalomastoma into three 

 sections : — 1st, Hainesia, which he considers to be possibly distinct 

 generically ; 2nd, Farcimen, Troschel ; and, 3rd, Megalomastoma 

 proper. In the first section, characterized by its oval aperture, 

 angular above, he places the Siamese M. Myersi, Haines, M. 

 croceum, Sow., and M. bifasciatum, Sow. The operculum is un- 

 known. In Farcimen he has included my Burmese M. gravidum, 

 together with other species provided with the normal thin cor- 



