240 STALK-EYED CRUSTACEA. 
According to A. Agassiz* the littoral Echinoid fauna of the Panama 
district is a mixed one, including generic elements from the adjoining dis- 
tricts. But the strictly Panamian species are with few exceptions representatives 
of the West Indian types. Jn a later workt the same author says that the 
principal differences in the Echinoid fauna on the two sides of the Isthmus 
are due to the immigration of true Atlantic types into the West Indian 
faunal region during the Tertiary and Post-Tertiary periods, after the Gulf of 
Mexico and the Caribbean Sea ceased to be in freer communication with the 
Pacific than with the Atlantic. — 
The total dissimilarity of the Coral fauna on the opposite sides of the 
Isthmus cannot be ignored.¢ It may be explained in part by the extreme 
sensitiveness of the reef-building species, such as flourish in the Caribbean 
Sea, to physical conditions. Mr. Agassiz § tells us that there could be no more 
striking contrast in topography than that between the Caribbean and the sea 
on the western side of Central America, with its abrupt continental slope and 
silt-covered floor. ‘To the enormous amount of silt that covers the ocean 
bottom, Agassiz attributes the absence of reef-building corals on the west 
coast, while Dana and others have ascribed it to the cold currents from the 
north and south that wash these shores, lowering the surface temperature 
at the Galapagos in the month of November, it is said, to 62° F. The affin- 
ity between the Miocene West Indian Corals and the recent species of the 
Pacific, which has been pointed out by Duncan || shows that the present dis- 
similarity is a result of the exclusion of the Pacific from the Caribbean Sea. 
Below the littoral zone there lies a belt, extending say from 150 to 
500 fathoms, which forms a sort of debatable ground, invaded on the 
one hand by littoral types from above, and on the other by characteristic 
deep-sea forms from below. Mingled with these are certain genera whose 
evolution finds its fullest expression in this intermediate zone. In illustra- 
tion: Cancer longipes and Plewroncodes monodon are shallow-water species of 
the coast of Chile which by descending into the cold waters of the interme- 
diate belt have been enabled to extend their range into the heart of the 
tropics. Paralomis is a genus of probable Patagonian origin which in a simi- 
lar way has worked northward in the cold waters of this intermediate bathy- 
* Mem. Mus. Comp. Zool., ITI. 221, 1872. { Mem. Mus. Comp. Zodl., X., No. 1, p. 79, 1883. 
£ See Verrill, in Proce. Boston Soe. Nat. Hist., X. 323, 1866. 
§ Bull. Mus. Comp. Zodl., XXIII. 9, 1892. 
|| Quarterly Journ. Geolog. Soc. London, XIX. 406-458, 1863; XX. 20-44, 1864. 
