272 Mr. W. King on the Tetrabranchiate Cephalopods. 



It will be seen that these shells are classified not only according 

 to their agreement with each other in general character, but also 

 in accordance with their relative order of creation. The synopsis 

 at the end will make this mode of classification intelligible at one 

 view. 



In a paper which I read at a meeting of the Natural History 

 Society of Northumberland, &c. in March 1843, on the family 

 Unionidce, the following view was advanced : — " I would again 

 urge attention to the two elements which are necessary to be 

 attended to in classifying any section of the animal kingdom, 

 namely, agreement of structural character and relative order of 

 creation. If all animated forms had been produced at the same 

 time, and there had been no previous dying out of these forms, 

 the plan that is generally adopted in arranging a zoological group, 

 that is, by attending to structural resemblances alone, would be 

 perfectly admissible ; but as innumerable species (for the most 

 part extinct) have succeeded each other during former periods of 

 the world's history, it follows, that to introduce such species in 

 a natural classification, without any reference to their order of 

 creation, would be equivalent to constructing a genealogical 

 chart without attending to the relative period of the different 

 families." Entertaining this view, it may be readily conceived 

 that I read with considerable interest the Chevalier Agassiz's ob- 

 servations, which have recently appeared, on the classification of 

 Fishes, and especially the following remark : — " It will not be 

 sufficient henceforwards to group genera and species according 

 to their organic affinities ; we must also take into account the 

 relative age of their appearance on the surface of the globe, and 

 the importance of each group in the different epochs of the ge- 

 neral development ; in a word, zoology ought 4o comprehend in 

 its systems the genealogy of the whole animal kingdom*." 



The Cephalopods have been divided into two sections, depend- 

 ing on their having two or four gills ; those with two gills are 

 termed Dibranchians, and such as have four are called Tetra- 

 branchians. This is Prof. Owen's arrangement and nomencla- 

 ture. The Calamary, Cuttle-fish, Argonaut, and Spirula are 

 examples of the dibranchiate, and the Pearly Nautilus represents 

 the tetrabranchiate section. 



The Dibranchians are extremely common at present ; as yet, 

 we have no evidence that they lived during any portion of the 

 primary period ; but from the abundance of their remains in the 

 corresponding rocks, it is certain that the secondary seas sus- 

 tained them in incredible numbers. The Tetrabranchians appear 



* Vide a translation of this paper in the last number (73) of the ' New 

 Edinburgh Philosophical Journal.' 



