﻿BRITISH FOSSIL CEPHALOPODA. 239 



some of its significance, though we may still argue that where the group has been 

 longest in existence there it will most abound, and the greater the abundance of 

 individuals the greater is their chance of preservation in the rocks. And this 

 observation loses none of its force when we notice that the comparative abundance of 

 Cephalopods in the Bala Beds of Britain, in proportion to the whole number of 

 Silurian forms, is much greater here than in Bohemia, where they swarm in Upper 

 Silurian, but are very sparse in Lower Silurian times. Perhaps another proof of 

 the earlier existence of the Orthoceras may be found in the fact that all the different 

 groups, even the most ornamented, are pretty uniformly distributed throughout the 

 series, and there is no apparent growth in complexity. 



With regard to the genus Cyrtoceras, it is noticeable that tbe endogastric group 

 antedates the exogastric. This is in harmony with the fact that the usually 

 endogastric genus Phragmoceras antedates the usually exogastric Gomphoceras. 

 With regard to the genus Poterioceras, the two species referred to it are so isolated 

 that it may well be doubted if it be of any value. Nevertheless, we may notice 

 that the forms of the Infiati, with imperfectly developed apertures, appear at the 

 commencement and at the close of the range of that group, and may indicate a 

 feebleness in its development. With regard to the Infiati generally, it is remarkable 

 how short-lived they are, coming in with rapidity and soon being comparatively 

 common forms, and then dying out suddenly and finally. Such great differences in 

 the history of one group as compared to another must surely indicate the action of 

 some peculiar cause which does not affect all groups alike. The position assigned 

 to the genus Ascoceras in the present work is consistent with their brief and late 

 period of existence, but any other position would not be. 



The same remark as to the endogastric forms preceding the others of the same 

 genus is true of the Nautilus, the only Lower Silurian species of which belong 

 to the subgenus Trocholites, with the siphuncle internal. Again and again does 

 the Nautilus appear to have tried this position in Trocholites, in Clymenia, and 

 in Aturia, and each time it has been a failure ; the external siphuncle has been 

 equally wanting in success. 



Of the other genera, the Trochoceras has the longest range, though represented 

 by different species in Upper and Lower Silurian rocks ; but none of these call for 

 any special remark. 



We come now to the " species." It is inevitable, in any profitable description of 

 a great group of life-forms, that we should make use of the specific nomenclature, 

 and limit each individual studied to the use of one or other of the names 

 employed, and thus assume at least the existence of definite groups, cut off from 

 all around them. This very necessity of description, however, relieves the describer 

 from having thereby expressed any opinion as to whether any such groups really 

 exist. The old idea of the independence of species — and their only, so to speak, 



