496 HYATT'S REVISION OF THE 



along the coasts of Europe and America, with regard to the temperature limiting the north- 

 ern spread of the genus Spongia. 



It -was entirely upon this gi-ound that I first began to doubt the accin-acy of the marine 

 isothermals for January of 62°, 56', 50°, which are laid down on Prof. Dana's map as com- 

 ing to a focus at Cape Hatteras. These should be carried down the coast, deviating inside 

 or west of the Gulf Stream to the southward, as does the more accurately drawn isotherm 

 of 68°, and should be drawn nearly parallel to the coast to some point south of Jackson- 

 ville for 62° ; to some point south of Savannah, probably Altamaha Sound, for 56° ; and 

 to Ca^DC Roman, north of Charleston, for 50°. This same rate of decrease would bring the 

 line of 44°, which ends at Cape Cod on Prof Dana's map, down the coast to a point a little 

 south of Cape Lookout, on the shore of North Carolina, and reduce the average tempera- 

 ture of the water at Cape Hatteras, during the month of January, to about 40°. 



This appears to be the result of the cold inshore current, which during the wdnter proba- 

 bly comes closer inshore, and combines with the atmosphere in affecting the surface water. 

 Such a current has been traced by the United States Fish Commission to the south of Cape 

 Cod, even in the shallow waters of the Vineyard and Long Island Sounds, and is constantly 

 referred to in the reports of the United States Coast Survey as coming to the surface farther 

 out to sea. This return current is crowded into a smaller space as it runs soutliward, and 

 would therefore be hkely to exercise a much more important influence proportionally to the 

 winter south of Cape Hatteras than to the north of that Cape, upon the surface waters in 

 January. Whether this is so or not, it is very remarkable that the selected limit for the 

 spread of all Keratose forms northward on the Amei-ican Coast was Cape Hatteras, and it 

 now turns out that the temperature of the surface water must be nearly the same as it is 

 along the northern limit of distribution of the same forms on the European side, namely, 

 a January isothermal of 40° on this side as compared with 41° on the other.^ It ought to 

 be observed here also, that Charleston is the most northerly point at which a Keratose form 

 has actually been found. This species, according to Prof McCrady's verl^al description, 

 must have had a very coai'se skeleton allied, if not identical, with Dysklea fragilis, which, 

 as previously observed, is the most northerly species on the European side also. It is 

 possible, therefore, that the generally unfavorable and sandy character of the coast may 

 have limited the distribution to a point as far south as Charleston ; but it would be unsafe at 

 present to adopt this view. Cape Hatteras being the natural limit of the entire littoral fauna. 



Dr. Buist ^ states that the temperature of the water in the Red Sea, between Aden and 

 Suez, varies between 65° and 85°. The same authority subsequently published^ a long 

 account of this region, accompanied by tables of temperatures from which the following 

 remarks were taken. The density of the water, except in the Gulf, was not excessive, 

 averaging about that of ordinary sea water except in the Gulf of Aden. The transparency 

 and purity of the water was very great, and this might also be remarked of the waters of 

 Florida and the Bahamas in the summer. Ten feet below the surflice the temperature at 

 Suez does not fall below 70°, or the air lower than 72° in January or February, and the heat 

 in summer is excessive, 80°, 82°, 92°, being not infrequently recorded at Aden, and 80°, 



^ See Dr. Carpenter's map. Proc. Royal Soc. Vol. xx. ^ Trans. Bombay Geogr. Soc. Vol. xiv, p. 3, 1859. 

 2 Trans. Bombay Geogr. Soc. Vol. ix, p. 39, 1850. 



