THE SESSILE BARNACLES. 19 



Species. 



Eastern North Atlantic and Mediterranean 1 18 



Western North Atlantic and Gulf of Mexico 11 



South Atlantic and Antarctic 4 



West coast, of South America 1 



Hawaiian Islands 1 



Southwestern Pacific (near Kermadec and Gambier Islands) 2 



East Indies (Malay Archipelago and Philippines to the Andamans) 13 



Total 50 



It -will be seen by the above table that (except in the North Pacific) 

 Verrucse are most abundant where the sea bottom has been most 

 assiduously explored, that is, in the North Atlantic. The compara- 

 tive scarcity of species in other seas may very likely turn out to be 

 due to the imperfection of our knowledge. 



While most deep-water Verrucas are now known from a single 

 station, the geographic distribution of species will, I think, eventually 

 prove to be somewhat extended, where conditions favor; this opinion 

 being based upon the data relating to V. alba, the varying forms 

 under V. calolheca, etc. The limits of their range seem to be deter- 

 mined by temperature, which of course is more or less closely corre- 

 lated with depth. Thus, V. alba, in its several varieties, has an 

 extension from. Barbados to Hatteras, yet always in bottom tempera- 

 tures of from 71.3 to 75 F., and depths of 45 to 68 fathoms. V. 

 calotheca, with the two forms I have segregated as races, has an even 

 greater geographic extension, with a known range in temperature of 

 only from 45. 6 to 48.3 F. Whether the restriction to a narrow 

 range of temperatures which seems indicated by the small number 

 of observations available be due to the direct effect of temperature 

 on the barnacle, 2 or to indirect action through the food supply or 

 other factors, is uncertain. That they are so restricted accounts for 

 the emphatic specific diversity I have found to exist between the 

 warm-water Verrucse of our continental shelf and the species obtained 

 by European expeditions in the eastern Atlantic. 



In the group of V. darwini, bicornuta, and ratJibuniana, which live 

 in 1,550 to 1,769 fathoms, bottom temperature 37 to 38.5 F., we 

 find very closely related species on the two sides of the North Atlantic 

 and as far south as the Antarctic Ocean. It seems likely that this 

 group of forms, living in the depths hi temperatures approaching 



1 Seven of the eighteen eastern Atlantic species were briefly diagnosed by Aurivillius, without figures. 

 Until a more complete account of these forms is given they can not be critically compared with really 

 known forms. Doctor Hoek significantly remarks that " Aurivilliusand Gravel have described 10 different 

 species of Verruca as occurring near the Azores; between some of these the differences seem to be very 

 small." It seems curious that the Verruca; obtained near the Azores by the Travailleur and Talisman 

 (described by Gruvel) should all be different from those dredged in the same waters by the Princesse A lice 

 (described by Aurivillius). 



2 Littoral and pelagic barnacles outside of the Tropics tolerate very considerable variations jij 

 temperature, 



