NO. 1 fraser: hydroids 5 



California only; and only one from the mainland coast south of south- 

 east Mexico. 



Of the 13 species that have been reported from South America far- 

 ther south than the area under consideration, 10 are cosmopolitan, one, 

 Plurnularia inagellanica, was obtained from Black Beach, Charles Is- 

 land, the station nearest to the Humbolt current, and another, Obelia 

 obtusidens, from Santa Elena Bay, the most southerly location at which 

 collections were made. The remaining species, Silicularia pedunculata, 

 has been previously reported only from South Georgia. Why it should 

 appear near Secas Islands, Panama, is difficult to understand. 



For the purpose of further studying the intrinsic distribution, the 

 area covered may be somewhat arbitrarily divided into three portions: 

 Division A, to include all the stations near the oceanic islands; Divi- 

 sion B, to include the mainland coast south of Balboa; Division C, to 

 include the mainland coast north and west of Balboa. 



Seventy-eight species were obtained in Division A, 60 in Division 

 B, and 115 in Division C. Eighteen species were found in each of the 

 three divisions; 2, in A and B only; 22, in A and C only; 21, in B and 

 C only; 35, in A only; 20, in B only; and 55, in C only. 



The distribution of species in the whole area, with the possible 

 exception of the northern portion of the coast of Lower California, 

 indicates strong affinities between the hydroid fauna here and that in 

 the North Atlantic, more particularly in the West Indian area. There 

 is evidence of some continuity of distribution in the California-Lower 

 California area but practically no indication of such continuity at the 

 southern extremity. 



The species found in this area and in the West Indian region must 

 be old species, existing as such since the earlier geological period when, 

 in the Panama region, there was no interruption in the distribution from 

 the Atlantic to the Pacific and vice versa. This long period of separa- 

 tion has given the opportunity for the evolution of the great number of 

 new species, indicated by the number described in this paper. 



In the area itself, the indication is that there is little interference 

 to distribution along the whole length of the mainland coast, or between 

 the oceanic division and the northwestern division of the mainland 

 coast, but close association is lacking between the hydroids of the 

 oceanic area and those of the coast of Ecuador and Colombia. 



In passing northward along the west coast of Lower California, 

 the most decided break in continuity of distribution in both fauna and 



