248 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



exclusively represented (Porites, Pavonia, Pocillopora, Dend.ro- 

 phyllia, Fungia, &c.). The West Indian coral fauna, on the other 

 hand, is deficient, either wholly or in great part, in many of the 

 more distinctive forms of the Pacific Ocean (Astraea, Pavonia, Pocil- 

 lopora, Fungia), a deficiency in part made good by a special devel- 

 opment of forms (Diploria, Agaricia, Siderina, Oculina, Cladocora, 

 Astrangia) which are either wholly wanting, or have only a rela-. 

 tively feeble representation, in the islands of the Pacific. The 

 madrepores of this region attain to prodigious size. Clumps of 

 Madrepora palrnata, a foliaceous species, have been found to meas- 

 ure two yards in width, while the branches of the tree-like M. 

 cervicornis not rarely reach a height of from ten to fifteen feet. 

 Professor Verrill has pointed out the somewhat remarkable fact that 

 none of the West Indian species of coral are specifically identical 

 with the species of the Panama coast, although most, if not all, of 

 the Florida reef-builders (species of Porites, Madrepora, Ma3andrina, 

 Manicina, Siderina, Agaricia, Orbicella) are also found on the coast 

 of Aspinwall. Doubtless this difference is in great part attribut- 

 able to the comparative brevity of the natural life of the species, as 

 it is well known that direct communication between the Atlantic 

 and Pacific oceans, in the region of Panama, was maintained during 

 the middle or later part of the Tertiary period (Miocene or Plio- 

 cene), and, if this was so, there can be little doubt that at that time 

 many of the forms on opposite sides of the present isthmus were 

 identical specifically. The present differentiation, arising from 

 isolation, would then date back at least as far as the permanent (or 

 nearly that) elevation of the separating land-mass, and not improb- 

 ably to a period considerably antecedent to that. For although, 

 as has already been seen, the number of recent species that extend 

 back to the Miocene period is very limited in most parts of the 

 earth's surface, yet just in the West India region Professor Duncan 

 has shown that very nearly ten per cent, of the Miocene coral fauna 

 is made up of existing species. This being true, we should nat- 

 urally expect to find, if the isolation of the Panarnaic and Gulf 

 faunas took place in the Miocene period, a considerable intermix- 

 ture of identical species, which is not the case. It appears prob- 

 able, therefore, that for some time previous to the final emergence 

 of the isthmus, whether through the down-wash of sediment or 

 otherwise, the region was in a measure rendered inimical to cora.l 



