804 BULLETIN OF THE 



genera having representatives in the great tropical helt surrounding 

 the globe, formed by the Indo-Pacific, Mediterranean, Senegalian, West 

 Indian, Panamic, and Polynesian fauna), — such as Cidaris, Diadema, 

 Echinometra, Tripneustes, Clypeaster, Stolonoclypus, Echinolampas, 

 Echinoneus, Brissus, the species of which have a great geographical 

 range, and are represented by the following species: — 



Cidaris metularia, Tripneustes sardicus, Echinometra lucunter, 

 Diadema Savignyi, Clypeaster Rangianus, Stolonoclypus placunarius, 

 Echinolampas oviformis, Echinoneus cyclostomus, Brissus carinatus, all 

 of which have an immense geographical distribution. 



The effect which currents play in shaping the geographical distribu- 

 tion of marine animals is very great ; we have an example in the Gulf 

 Stream and the northern branch of the Amazonian current flowing into 

 the Gulf of Mexico, which account fully for the great range of the 

 more common littoral species. The Japanese current makes itself felt 

 as far as San Diego, two species of Echini extending in the Northern 

 Pacific from the northern part of Japan along Kamtchatka, the Aleutian 

 Islands, Sitka, Vancouver's Island, the one as far as Cape Mendocino 

 (T. drobachiensis), the other (Dendraster excentricus) to San Diego. 

 The Indo-Pacific equatorial current has undoubtedly been the main 

 agent of the extensive geographical range of such species as Cidaris 

 metularia, Echinoneus cyclostomus, Heterocentrotus mammillatus, Dia- 

 dema Savignyi, Tripneustes sardicus, Echinolampas oviformis, Brissus 

 carinatus, Stolonoclypus placunarius. 



The effect of currents in thus extending the distribution of marine 

 animals would act very differently upon the several classes of the 

 animal kingdom, and its efficiency depends to a great extent upon the 

 nature of their earlier stages, and upon their habits during that period. 

 The time during which the Pluteus of Echini remains helpless at the 

 mercy of the currents is considerable : from early spring till late in the 

 summer is the usual time required for the full growth of the Pluteus in 

 many species of Sea-urchins, and the distance which the young could thus 

 be transported, even by a sluggish current, during a single season, must 

 be considerable, even under the most unfavorable circumstances. 



Various writers have attempted to retrace, in former geological 

 periods, the probable course of the currents and their effect upon the 

 geographical distribution of marine animals ; they all agree in repre- 

 senting up to the cretaceous period an unbroken equatorial current, 



