NORTH AMERICAN PORIFER^. PART II. 499 



isotherm for the coldest months of about 50°, and possibly lower to some as yet undecided 

 limit, or the equally injurious heat shown by an average for the same month of about 65° 

 and upwards, to 80°.^ 



There is a darkening of the color of the fibres about the base, and frequently of the 

 whole sponge, which may occur with any of the inferior qualities in any cold climate, or 

 unfavorable situation, as at Istria, and varies also with .the age and size of the specimen. 

 These influences, however, never produce so marked an eifect as in a hotter climate, nor 

 does the deterioration of the fibre and of the density of the skeleton go so far ; neverthe- 

 less, the Nassau sponges, which are lighter colored than the Gerbis sponges, and the 

 remarks above on the influence of suspended matter near the shore, seem to point out very 

 conclusively, that heat does not control the color entirely, though it may largely influence it. 



Another point in this connection is, that the deepest color is always in the interior, and 

 the lightest colored parts are external, in the position most exposed to the action of li'dit ; 

 and this, though not necessarily, is probably the hottest part of the organism durins: the 

 heated term of the year in the shallower waters, where the darkest colored forms are 

 mostly found. It has been suggested by Eckhel that this coloration was due to the 

 presence of iron in the sediment or sea bottom, but this could hardly be the case in the 

 vicinity of coral reefs. In fact, dark internal coloration appears to result from, or correllate 

 with, the deterioration of the skeleton as an internal change in structure, which varies with 

 the species, the age and the health of the specimen, and probably with the chemical com- 

 position of the fibres themselves. 



If we now, in conclusion, turn our attention to the general limits of the distribution of 

 the Keratosa as a whole, we find that the extreme points already mentioned may be all 

 inclosed within a central zone extending on both sides of the equator to the isothermals for 

 the coldest months in the year of 40° Fahr., and these, therefore, would approximately 

 represent the iiatural boundaries of the circumterraneous area occupied by the order. 



This distribution does not seem explicable in any way unless we assume, that the 

 Keratosa originated in North America, and have spread in all directions from this area. 

 This hypothesis also appears to me to explain more satisfactorily, according to our present 



1 A collection just received from the Museum of Yale southwest of Auklaml Islands, near the Macquarrie Islands, 

 College contains a specimen of Spnnr/ia diicus, var. aukli- in lat. 55^, long. 157°, the surface temperature in January 

 nensjs, from the Aukland Islands, south of New Zealand, varied between 31° - 44°, an average of 3 7°. 5, and in lati- 

 latitude about 50° 20', and longitude 167°, and therefore if tude 57° 52', with nearly the same longitude as the Aukland 

 this locality is correct, affording an instance of a true Spon- Islands, the thermometer registered 42° in December. Un- 

 gian form living in water, according to Dana's map, with a fortunately these temperatures were taken during the hot- 

 July isotherm of about 41°. I was led to doubt the accu- test months, and thus while 41° may be a very o-ood average 

 racy of this isothermal line by this fact; but the examina- for the surface in this region during the month of July, and 

 tion of Mr. J. Prestwieh's Memoir on " Submarine Temper- may serve the purposes of a general map, like that of Prof, 

 atures," in Trans. Royal Soe., Vol. 165, Pt. 2, 1876, which Dana, it does not accurately express the isotherm for the 

 has just come to hand, confirms Dana's view. The tem- coldest months in the year in the shallow waters of the Auk- 

 peratures for the surfitce recorded in the neighborhood of land Islands, which appear to lie to the east and north 

 New Zealand, on the parallel of Dana's isotherm for July of the colder area just south of Van Diemen's Land, and it 

 of 50°, are as follows: In January, near the west coast of is possible that they may average some degrees higher. 

 South Inland, at 534 feet, 65°. 8, and surface 63°; in Cook's The only indubitable exception, which I have found, to 

 Straits, surface 63°. 5 to 64°.4, off' the southeast coast of this (50°) limit of temperature, occurs also in this collection 

 North Island, in February 65°. 2 for surface, and 58°. 7 at — a specimen from Ft. Macon, of Spongia vermiculata. It 

 depth of 506 feet. Near Antipodes Island, on parallel of is one of the coarsest, most open, and aberrant forms, but 

 49°. 17 considerably to the east and a little north of Aukland is a true Spongian, and the isotherm of the surfiice water for 

 Islands, the temperature of surface was found in December Janu.ary of tliis locality cannot be placed higher than 44°. 

 to be 53° in six observations by Sir James Ross, and to the It is also of a light color. 



