GASTEROPODA. 43 



inhabited a deep-water portion of tlie Crag sea, is equivalent to a diminished temperature ; 

 while this tropical representative must have frequented a shallower portion, having a 

 more elevated temperature, that is, if the law be admitted that hnes of depth are equi- 

 valent to zones of latitude, or isothermal lines. This theory will not satisfactorily 

 explain how these arctic and torrid representatives are quietly reposing together in 

 the Crag beds without the intervention of disturbing causes, of which there are cer- 

 tainly no indications in the locality from which they were obtained. Whatever may 

 have been the temperature of the Coralline Crag sea, — and I think it may have been 

 rather more elevated than that of our present seas, — it is evident that these animals 

 have now retired or migrated into those parts of the world, the one north and the other 

 south, where the temperature of both is very different from that which must have been 

 favorable to their existence at the period spoken of, and they have, therefore, in some 

 degree changed their nature in assimilating such extremes to their present existence. 

 Tlieir mode of dispersion was, it is presumed, by means of currents, which perhaps 

 had at that period a northerly direction, thus dispersing those species which are now 

 considered as arctic forms, while the torrid representatives might have died out where 

 they are now found, and their dispersion to the southward may have been by 

 southerly cuiTcnts from the contemporaneous seas, producing the Touraine beds. 

 Conclusions regarding the temperature of the sea during the period the Crag was 

 deposited have been drawn from the presence of such animals as Pyrula, Pholadomya, 

 &c., and an elevation considerably above that of the British seas of the present day has 

 been assigned to it at that period in consequence, while I believe the change only to 

 have taken place in the animals themselves; and this might arise from their acquiring 

 habits of enduring increased or diminished temperature by gradual migration, until 

 extended location had caused them to reach such extremes as would have been fatal 

 to their existence had their removal been suddenly effected. 



Trophon,* BeMontf. 1810. 



FtJSUS (spec.) Bruguiere. 

 Tritonium. Mailer. 1773. 

 Atractus. Agassiz. 1840. 



Gen. Char. Shell fusiform and turreted, sometimes ventricose, with many rounded 

 volutions, costated, rarely smooth, often striated ; aperture terminating in a moderately 

 elongated canal ; outer lip simple ; columella smooth ; operculum corneous. 



As the name of Fiisus may be more correctly reserved for those species of which 

 Murex porrectus. Brand., would form the type, I have used the above, proposed by 

 De Montfort, 1810, and adopted by Moller, 1842, in preference to Tritonium, Miiller, 

 1773 ; the latter being objectionable, on account of two names of similar import being 

 extensively known as generic terms in the same class, namely, Tritonia and Triton. 



* Etym. ? 



