kofoid: mutations ix ceratium. 241 



expect ill typical seasonal changes in Ceratium that a larger proportion 

 of the individuals would be affected by the general climatic factors which 

 underlie such modifications. As the following table, taken from Loh- 

 mann's paper, shows, the total numbers of all the short-horned "forms " 

 constitute but a small portion of the numbers of the type C. trijJOshaUi- 

 cum, not exceeding 1 per 100 in 7 of the 15 collections between August 24 

 .and November 29 and rising above 12 in but one instance, while only be- 

 tween September 21 and October 4 do the totals of the " forms" rise 

 above one eighth of the numbers of the typical 6'. fnpos balticum. 



Their increase in numbers is (see table) preceded and accompanied 

 by considerable fluctuations in the temperature, salinity, and specific 

 gravity of the sea water. The conditions on August 24 represent the 

 cessation of a rapid fall in temperature, 3° in a fortnight, increase in 

 salinity (1.415 to 1.669), and specific gi'avity (9.26 to 11.79), with a less 

 rapid increase in deeper (15 m.) waters (1.936 to 2.04-3). During the 

 period August 24 to September 28 surface waters cool slowly to 1 3.9°, but 

 deeper waters rise to 15.4° on August 31 and fall slowly during tlie follow- 

 ing month. These changes are possibly the result of the Baltic cir- 

 culation, and in comparison with other conditions at other seasons in the 

 table are unusual. Change is indicated in the character of the water. 

 Lohmann's tables therefore exhibit to an unknown extent the effect 

 of this circulation, and the changes in numbers of the organisms under 

 discussion may in part (how large can only be conjectured) be due to the 

 inflow of water of higher salinity and a diff'erent plankton content. A 

 hydrographic factor introducing a different plankton rather than merely 

 a seasonal one modifying an existing plankton is suggested by these 

 physical data as operative in producing the changes shown in the relative 

 numbers of the organisms listed in Lohmann's statistical tables. 



The same hydrographic causes probably underlie the occurrence in 

 the winter plankton of the Baltic of Ceratmm tripos balticum forma 

 pendida and f. penduloides Lohmann ( = C. trijjos f. hiemale Paulsen). 

 This is a long-horned C. tripos, probably entering the Baltic in winter, 

 as Lohmann suggests, with the invasion of waters of higher salinity 

 (Gulf Stream 1). Though it has a seasonal occurrence in the Baltic, it 

 is probably not everywhere a " seasonal " form, as Paulsen's name and 

 Lohmann's discussion seem to indicate, but an invader from warmer 

 seas, where it is not uncommon. In Baltic waters abnormal conditions 

 (see Lohmann's figure 21 I g, h) arise in the horns as the result of the 

 changes to which it is subjected as the invading waters mingle with 

 those of lower salinity. 



