REPORT ON THE ISOPODA. 83 



the northernmost station in comparatively shallow water (Station 120, 675 fathoms); this 

 specimen is, however, distinctly smaller than any of those obtained in deeper water at 

 Stations 146 and 147. 



It must be remembered, of course, that Gerstaecker's statements as well as mine 

 depend after all upon very few facts; it would be extremely rash at present to insist 

 upon any such generalisation as has been put forward by Gerstaecker in the work already 

 quoted, but it seems worth while to call the attention of naturalists to the facts such as 

 they are. 



The remaining deep-sea species were dredged close to the east coast of South America ; 

 Serolis gracilis from a single Station (Station 120), off Pernambuco, in 675 fathoms, and 

 Serolis necera from two Stations close together and a little farther to the south, off Buenos 

 Ayres, at Stations 320 and 318, in 600 and 2040 fathoms respectively; 2040 fathoms is 

 the greatest depth which the genus is known to inhabit. 



It appears therefore that the deep-sea forms of the genus, although not absolutely 

 confined to the neighbourhood of the great continents, attain to their greatest development 

 both in number of species and individuals in this situation, and are never found at any 

 distance from some land — continent or oceanic island. 



The genus Sei'olis has evidently originated in the southern hemisphere, probably 

 round the shores of the south polar continent, and has thence spread northwards, its 

 range being apparently limited by temperature ; accordingly we find that in every case 

 those species which occur near the equator [Serolis gracilis, Serolis antarctica, Serolis 

 necBra) occur there in deep water where the conditions, as far as temperature is concerned, 

 are not so different from the conditions which must obtain on the shores of Kero;uelen and 

 Patagonia ; the one fact, however, which seems to militate against such an hypothesis is the 

 occurrence of Serolis carinata in shalloiv water as far north as San Diego in California. It 

 must be remembered, however, that the temperature of that portion of the Pacific is not so 

 high as might be expected from its latitude ; a cold current from the Antarctic area sweeps 

 along the western shores of South America, and the existence of this current has perhaps 

 rendered it possible for Serolis carinata (or its ancestors) to migrate farther to the north 

 than would be possible, for example, on the eastern shores of the same continent ; more- 

 over, a glance at the map of the world will show that here alone is there any du'ect 

 land communication between the area occupied by the shallow-water species of the genus 

 Serolis in the southern hemisphere and the more northern regions ; elsewhere tracts of 

 deep water have possibly aided in preventing their access to the equatorial regions and 

 the northern hemisphere, though it seems more probable, from what has abeady been 

 said, that the distribution of the group has been more restrained by conditions of tem- 

 perature than by any other cause. 



Comparing the deep-sea with the shallow- water species of Serolis, it appears (1) that 

 the genus is pre-eminently a shallow-water genus, the number of deep-sea forms being 



