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 feio exceptixms repi'esentatives 

 of the West Indian types. In a later work t 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 tlie 

 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 tliat the present dis- 

 similarity is a result of the exclusion of the Pacific from the Caribbean Sea. 



Below tlie 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 lomjijjes and Plcwrmcodes 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 pi'obable Patagonian origin which in a simi- 

 lar way has worked northward in the cold waters of this intermediate bathy- 



• Mem. Mils. Cnmp. Zoot, III. 221, 1S72. f Mem. Mus. Conip. Zool., X., No. ], p. 79, 1883. 



X See Verrill, in Proc. Boston Soc. Nat. Hist., X. 323, 1S66. 



i Bull. Mus. Comp. Zool., XXIII. 9, 1892. 



II Quarterly Journ. Geolog. Soc. London, XIX. 400-438, 1863; XX. 20-44, 1864. 



