236 PANAMIC DEEP SEA ECHINI. 



species belonging to eight genera, all of which are represented in the I 



West Indies. Of the twenty-four West Indian continental genera thir- ^ 



teen are not found in the Panamic district, though of these thirteen no 

 less than eleven are found on the continental slope of the East Indian 

 Archipelago. Of the Panamic genera only three are not found in the 

 West Indies. 



When we come to the abyssal realm in the West Indian area there are six 

 species belonging to as many genera; on the Panamic side there are 13 

 species representing ten genera ; of these only five are also found in the 

 abyssal West Indian realm, though two of the Panamic abyssal genera occur 

 on the continental West Indian slope. 



It must be noticed that two of the genera occurring in the continental 

 slope of the West Indies are, as thus far known, abyssal genera in the 

 Panamic realm. They are Dermatodiadema and Phormosoma. 



Such a close correspondence in the distribution of the species on the two 

 sides of the Isthmus of Panama would seem to indicate, between the West 

 Indian and Panamic realms, a very free communication in comparatively 

 recent geological times down to a depth of about 100 fathoms, the repre- 

 sentative species on the two sides of the Isthmus belonging to genera with 

 a limited bathymetrical range. 



The species of the continental slope belong to genera with a more ex- 

 tended bathymetrical range, many dating back to the Cretaceous and 

 Jurassic periods. Of the twenty-three West Indian genera only six, Doro- 

 cidaris (Centrocidaris), Porocidaris, Salenia, Linopneustes, Brissus, and 

 Periaster, occur also on the Panamic side, though two additional genera 

 are abyssal Panamic while they are continental West Indian : Dermatodia- 

 dema, Phormosoma. 



The number of West Indian continental genera also found in the East 

 Indian Archipelago is very striking. They are Porocidaris, Coelopleurus, 

 Asthenosoma, Gcnocidaris, Trigonocidaris, Ilemipedina, Pala?otropus, Lin- 

 opneustes, Neolampas, Hemiaster, Rhynobrissus. 



Nacospatangus and Plexechinus are Panamic. 



The nine continental genera indicate a connection across the Isthmus of 

 at least 250 fathoms in depth. 



When we take the abyssal genera, the allies of which date back to the 

 Jurassic period, there are five of the six West Indian abyssal genera, Pour- 

 talesia, Urechinus, Cystechinus, Homolampas, and Aerope, which are also 



