160 The Commercial Products of the Sea. 



This result, in which three species appear on both sides of 

 the Atlantic as representing alone the marketable qualities 

 of the genus Spongia, becomes of double interest when 

 these varieties, or local species as they might be called, are 

 compared one with another. It is then found that the 

 aspect of the surface is closely similar in each of the three ; 

 that sub-species tubulifera represents Spongia officinalis, 

 sub-species gossypina offsets Spongia equina in the same 

 way, and lastly, sub-species corlosia has the same relation 

 to Spongia agaricina. In order to make it still more con- 

 vincing that such a relationship is not the result of an 

 artificial arrangement, it becomes necessary to describe 

 some of the facts more at length. First, their similarities 

 of surface and aspect are precisely the same as those which 

 experience has led me to adopt in' the designation of 

 species in this group. Secondly, their differences can be 

 accounted for by the difference in habitat, and are of varietal 

 and not of specific value, according to the accepted use of 

 the term species. 



The whole group of Keratosa is confined to seas in 

 which the differences observable between the winter and 

 summer isotherms are not excessive. None are found 

 north of Cape Hatteras and Bermuda, and doubtless a 

 similar limit occurs to the southward of the equator ; at 

 least, it is a noticeable fact that the only specimens in the 

 Museum of Comparative Zoology are from the island of 

 Fernando Noronha. On the Pacific shore, Southern Cali- 

 fornia and Chili are the extreme points so far known. On 

 the opposite coast of the Atlantic they are recorded from 

 England to the Cape of Good Hope, and also at the island 

 of Teneriffe. In the Indian Ocean they are found all 

 along the east coast of Africa, at the Mauritius, and on the 

 shores of India. They have been described from the 



