MUSEUM OF COMPARATIVE ZOOLOGY. 



315 



Caribbean Fauna. 



Ophiozona impressa Lyia. 

 Opbiocoma pumila Ltk. 

 Ophiactis Krebsii Ltk. 



Amphiura tenera Ltk. 



Ampliiura Riisei Ltk. 

 Ophiophragmus septus Lym. 

 Ophiocnida scabriuscula Lym. 

 Hemipholis cordifera Lym. 

 Ophionereis reticulata Ltk. 

 Ophiothrix violacea M. and T. 



Panama Fauna. 



Ophiozona pacifica Lym. 

 Opbiocoma Alexandri Lym. 

 Ophiactis virescens Orst. and Ltk. 

 Amphiura violacea Ltk. 



" puntarense Ltk. 



" microdiscus Ltk. 



Amphiura grisea Ljn. 

 Ophiophragmus marginatus Lym. 

 Ophiocnida hispida Lym. 

 Hemipholis affinis Ljn. 

 Ophionereis annulata Lym. 

 Ophiothrix spiculata LeC. 



How is it that the vast Pacific fauna, common to the waters between 

 Zanzibar and the Sandwich Islands and between Loo Choo and the 

 Kingsmill group, changes its character near Panama, and takes on a 

 •partial Caribbean form ? We might think that the mingling of the 

 two oceans, before the upheaval of the isthmus, was the origin, and 

 that the differences between these species was the measure of their 

 variation since the cretaceous period. But then the Caribbean forms 

 appear on the Pacific side, while the Pacific forms seem not to come 

 over ; and no matter whether there is or is not a difference of level 

 between the oceans, it would scarcely have availed to prevent a mix- 

 ture in both directions by storms, or by currents. It is also perfectly 

 credible that water-birds should mix the faunas across an isthmus 

 which has a minimum width of twenty-eight miles, just as they con- 

 vey fish eggs to distant and isolated ponds. But again there is the 

 same objection as before. I must therefore content myself with saying 

 that of these twelve pairs of species there are several that would prob- 

 ably be considered only as varieties, if they lived in the same waters. 

 Speculation is, after all, of small value, because the facts are insufficient, 

 and because there is a prospect of getting many more facts. For ex- 

 ample, all the diligent dredging on the European coasts had failed to 

 show a species of brittle-star identical with the Caribbean ; but, almost 

 at the same time, two expeditions bring up, from a depth of only 75 

 fathoms, a species new to science and common to the two sides of the 

 warm Atlantic. Such is the value of negative evidence ! 



