xlii 



FACTS AND OBSERVATIONS. 



stained, abraded, or greatly damaged. The facies of the new neighbourhood and its fauna may be 

 inconsistent with the idea of the new fossil being a native. 



In the power of migration the Mollusca vary greatly. Many mollusca, fixed during the early 

 portion of their life, are free to rove at pleasure afterwards, and vice versa. They travel much in 

 the ova, or as fry. In the Lake of the Woods (Rupert's Land) great numbers of minute, yet 

 delicately elaborated, trilobites were found in a Ludlow rock. The same occurred in Wales to 

 Mr. J. W. Salter. These creatures were easily transportable. 



The sum of migratory power seems pretty equably distributed among the several orders, except 

 the Echinodermata and one or two more, the sources of this power, it is to be recollected, being of 

 different kinds. 



The Brachiopoda, Coelenterata, and other sea-life are swept away by currents, and take their 

 chance of survival at new stations. 



The oceanic free swimmers, Cephalopoda, Pteropoda, &c. travel far and near, Gasteropoda 

 beginning life in the Pteropod form (E. Forbes, Edin. Phil. Journ. xxxvi.). 



The many modes of migration over sea-bottoms, active and passive, are well known. 



The faculty of sustaining great bathymetric range must essentially assist the migrant ; and it was 

 common and powerful in the Silurian epoch, as we learn from the numerous " grounds " frequented 

 by many of its faunae. This is well shown in Table xix. (Quart. Journ. Geol. Soc. Lond. vol. xv. 

 p. 315) *, as corrected by Mr. J. W. Salter. Together with the accompanying text this Table 

 contains much matter concerning Silurian nature, which need not be repeated here. 



Here that portion of the fauna which, from their wide distribution, have been called " uni- 

 versal " are found in eight, nine, and ten different sediments. In proof of this I subjoin the 

 following little Table. It is merely a selection of some of the more striking instances in the 

 several orders, neglecting the number of sediments below seven. Where the mollusca can 

 exist in health on many sediments, they can travel far ; for considerable changes of depth are 

 implied. 



TABLE T. 



Mollusca, except free swimmers, are slow travellers ; but like the Crustacea, with a sufficient 

 allowance of time they move to great distances when assisted by shore-lines and some steadiness of 

 ocean depths. The fauna of the north-east coast of North America, from Nova Scotia to Virginia, 

 is nearly the same. 



The directions taken by Silurian mollusks in leaving their birthplace must have often depended, 

 as now, on oscillatory movements. They had to follow the level most advantageous to them ; 

 neglecting this they mast have suffered, and perhaps perished. 



The Silurian areas in both the New and the Old Worlds were so extraordinarily large that 

 their inhabitants had very free range. The directions taken by the majority are easily obtained from 

 the 'Thesaurus/ through a knowledge of their points of origin. 



There are many modes of treating the subject of direction in migration. We have already 

 taken Bohemia as a fixed point of centre, and ascertained the directions from which, and to which, 



* On the habitats of the fauna of Wales. 



