GASTEROPODA. 43 



time inhabited a deep-water portion of the 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 

 lines of depth are equivalent 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 certainly 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 assimi- 

 lating such extremes to their present existence. Their 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 currents 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. 



Trophox,* Be Mont/. 1810. 



Fusus (spec.) Bruguitre. 

 Triton ium. Mutter. 1/73. 

 Atkactus. Agussiz. 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 Fusus 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. 



* Etvra. ? 



