SUBMARINE DEPOSITS. 285 
want of light, etc., seemed to militate against this supposition. But 
that animals living near the surface contribute also a not inconsider- 
able proportion, is proved by the numerous shells of pteropods, occa- 
sional teeth of fishes," etc. 
It was on this bottom that Pourtalés dredged the interesting 
little stalked crinoid Rhizoerinus, which he referred at first to 
Bourgueticrinus. The importance attached to the subsequent 
discovery of the identity of Bourgueticrinus with the Rhizo- 
crinus dredged by Sars off the coast of Norway, as well as 
by Pourtalés, Agassiz, Thomson, and Carpenter, undoubtedly 
awakened the general interest of naturalists to the problems of 
thalassography, and to the two successful hauls of the dredge 
in the Straits of Florida and off the Lofoten Islands we owe in 
great part our recent deep-sea explorations. As stated by Pour- 
talés, — 
“ The sandy bottom ends exactly at Cape Florida. Key Biscayne, of 
which the southern point is Cape Florida, eonsists in great part of 
siliceous sand. The next island to the southward, only five miles dis- 
tant, shows no trace of sand, but consists exclusively of coral lime- 
stone, of which the whole range of the Florida Keys is also formed. 
* About Cape Sable siliceous sand reappears, and extends along the 
western coast of Florida, though at first strongly mixed with lime. 
“Tt is remarkable how the littoral fauna changes with the constitu- 
tion of the bottom. Many forms of animals peculiar to the Caroli- 
nian fauna disappear at Cape Florida, and reappear at Cape Sable and 
on the west coast of the peninsula. Between these points they are en- 
tirely erowded out by the interposition of the West Indian fauna of the 
coral reefs. To take but one example, oysters are not found on the 
coral bottom, though abundant to the east and west on the sandy 
bottom." 
This shows that the nature of the bottom far more than the 
depth is the cause of the abrupt changes of fauna which are 
observed when we pass from one kind of bottom to another. 
To the westward of the shores of the southern extremity of 
Florida we gradually come upon the belt of mud, more or less 
mixed with caleareous matter, which stretches from Florida 
towards deep water and around the shores of the Gulf of Mex- 
ico, until it strikes again the calcareous deposits of the Yucatan 
