REPORT ON THE ECHINOIDEA. 



239 



When we come to the strictly deep-sea species, we find that many of them encroach at 

 their upper limit into the lower limit of the continental species. Only a comparatively 

 small number' of species can be called abyssal, the lower limit of these not being, how- 

 ever, any greater than that of many of the species which have a higher continental 

 limit. 



There are fifty deep-sea species, of which twenty-seven either approach or encroach 

 upon the lower limits of the continental species. Of these forty-six there are twelve which 

 extend to a greater depth than 2000 fathoms, 2900 fathoms being the greatest depth at 

 which any Sea-urchin has been dredged by the Challenger; fifteen other species extend to a 

 greater depth than 1000 fathoms and less than 2000. Of the twenty-four species which 

 approach or extend to the lower continental limit, five extend to beyond 2000 fathoms, 

 two to beyond 1500 fathoms, and three to over 1000 fathoms. The abyssal species 

 are distributed as follows : — 



