106 PROCEEDINGS OF WASHINGTON MEETING. 



many of thegenera arc represented by equivalenl species un the two sides of Centra 

 America is clear proof of the former connection between the waters of the Antillean 

 and Panamaic regions ; but the resemblance between the echinoidea of these two 

 provinces seems to be less close than is that between the Mediterranean ami West 

 Indian Miocene. No one species of echinoid is common to both shores of Central 

 America, and the representative species are often more distinct than those of the 

 two Miocene faunas. Hence if Professor Agassiz is justified in his conclusi* >n of the 

 common origin of the Antillean and Panamaic echinoidea, then so also must the 

 Antillean and West Indian Miocene faunas have been derived from a common 

 source. And just as it is considered to prove in the one case a depression of Cen- 

 tral America which brought the waters of the Pacific and the Caribbean into con- 

 nection, so in the other case we must assume a period of elevation which produced 

 a hand of shallow sea across the mid-Atlantic. Whether it he assumed that the 

 fauna originated in the Mediterranean and migrated to the West Indies, or vice 

 versa, or whether it developed in some area in the Atlantic now deeply submerged, 

 this shallow water connection is essential. 



But there are two explanations that might be proposed that could not involve 

 any such complete opposition to the theory of the permanence of the ocean basins. 

 It might be urged (1) that the common element in the two faunas worked its way 

 around from the one area to the other along the shallow northern shores of the 

 Atlantic; or (2) that the connection was established by the free-swimming larval 

 forms. But we are not without evidence against both of these hypotheses. If we 

 follow the Echinoid fauna of the Helvetian (middle Miocene) from its typical de- 

 velopment in Egypt, Malta, Sicily, and Italy toward the north we find at the most 

 northerly area in Brittany that though a considerable series of echinoids remain, 

 the group of species and genera which ally the Mediterranean to the West Indian 

 fauna has completely disappeared. It is just the same in America; the Miocene 

 of South Carolina has yielded none of the samegroup, which is replaced by species 

 of Mettita, Encopt . Echinocardium, etc. This fauna has resemblances to the West In- 

 dian, but it is by an element not typically represented in the Mediterranean. Thus. 

 on both sides < if the Atlantic the evidence seems fairly conclusive that the migration 

 did not follow the northern route. But we are fortunately not compelled to rely 

 on negative evidence alone. In the Azores, in Madeira, and in the Grand Canary 

 there are Miocene beds which have yielded a small echinoid fauna ; in each case 

 the species when determinable are found to be those characteristic of or close allies 

 to the Mediterranean Miocene; in some cases the species are represented by the 

 same varieties. This is, of course, proof only of the original extension of the Med- 

 iterranean fauna as far west asthe Azores, but this is a very considerable step across 

 the Atlantic; and some West Indian forms, as Temnechinus, occur elsewhere only at 

 the Azores, and thus serve to show the completion of the bridge. 



In regard to the second hypothesis explaining the connection by the free-swim- 

 ming larva:' it may be objected that the chances of so delicate an organism as a 

 pluteus surviving the journey across the Atlantic must be somewhat remote, and 

 the species would have no chance of establishing itself unless a number of the 

 plutei arrived simultaneously at a suitable locality. I do not remember that the 

 Challenger surface nets ever collected any plutei of a littoral species in mid-ocean. 

 But here again we are fortunately not left to decide on mere probabilities such as 

 these. Many living echinoidea are now known to be viviparous and to have no 

 free-swimming stage. Now- Schizaster parkinsoni has in a very marked degreeall the 



