NORTH AMERICAN PORIFER^. PART II. 493 



ranean, they are never for any length of time exposed to a lower degree of cold than 50° 

 to 55°, and that this produces a very unfavorable effect upon the quality of the skeleton. 



This of course directly negatives the conclusions drawn from the conditions found to 

 exist on the southern coast of the Western Basin, as contrasted with those of Sardinia, 

 Sicily, and southern Italy, and it becomes evident that the temperature alone cannot be the 

 cause of the absence of the Spongise from these shores. 



Locally we can say with some degree of probability, that the temperature and the fresh 

 waters of the Po, with the vast quantity of silt which is carried by it to the sea, and 

 distributed along the shore to the southward, have jJi'oved to be barriers in this direction. 

 When, however, the attempt is made to explain Avhy they are absent from all the other 

 northern shores of both the islands and the coasts of the Western Basin, all single causes 

 seem to fail, and I can only suggest the following, viz., that, where the limits of temperature 

 are neared, the absence of small islands, or very slight local peculiarities, may occasion 

 the absence of the commercial sorts. It must also be remembered that it is not distinctly 

 known that all are wanting ; our information is limited wholly to the commercial sorts, 

 and the other coarser and non-marketable varieties have not yet been systematically hunted 

 for by naturalists. 



The finest sj^onges in the Mediterranean, those of the Levant and oif the Syrian and 

 Tripoli coasts, are found between the average aerial winter temperatures of 63° and 70°, 

 and the isochrymals of 50°-57°, and the table (p. 489) shows that at no time of the year 

 are these, which, as stated by Von Eckhel, occiu- in the deeper water at a distance from the 

 coast, probably exposed to a lower temperature than 60". 



This repeated approximation of all calculations of the surface temperature, however 

 made or wherever made in the winter, to a marine isochrymal of 55° to 60°, although not so 

 convincing as a series of actual observations, appear to point this out as probably a close 

 approximation to the actual isochrymal which limits the geographical distribution of the 

 prime qualities of commercial sponges, and this view derives great additional suj^port from 

 the flxct, already stated, that this is the constant temperature of deep water in summer, 

 and the probable isochrymal at the depth of 30-50 fath. in winter in a large part of the 

 Mediterranean. 



I am indebted to the U. S. Signal Service Office for a series of tables giving the average 

 temperature for all the months in the year of the surface water of St. Marks, on the west 

 coast of Florida, lat. 28° 55', long. 84° 07' ; Punta Rossa, also on Avest coast, in lat. 26° 29' ; 

 long. 82° 10'; Key West at the exti*eme south, and Jacksonville on the Atlantic side. The 

 following is a copy of the average temperatures of all the months in the year. The St. 

 Marks' column, however, is taken from only one complete series of observations ; while the 

 Punta Rossa, Key West and Jacksonville averages are all deduced from three years of con- 

 tinuous observations. 



The isotherm for the month of January at St. Marks is 63°. 5, for Punta Rossa, 68°. 8, 

 and for Key West 73°. Anchor Key and Cedar Keys, famous localities for spongers, are 

 between St. Marks and Punta Rossa. These temperatures differ but a few degrees from the 

 aerial isotherm, which is at St. Marks 5°, at Punta Rossa 10°, and at Key West 4°. 5 colder 

 than the water, whereas in summer the temperature of the air at these localities is re- 

 S23ectively 1°.5, 6° and 5" v/armer than the water. The marine isotherm for the month 



MEMOmS BOST. SOC. NAT. HIST. TOL. H. 124 



