42 



KR. BONNEVIE. 



[REP. OF THE "MICHAEL SARS" NORTH 



American coast in an easterly direction. The coincidence 

 in the occurrence of both organisms, would in each case 

 find a simple explanation. 



Of these two possibilities the latter seems to me the 

 most probable, remembering that the species of Limacina 

 have been found also in places where a connection with 

 the Sargasso weed has not been observed, as for example 

 along the African coast. The three warm-water species of 

 Limacina may in some way or other be biologically 

 connected with the coastlines on both sides of the North 



It will be seen from this table, that most of them 

 must be considered surface-species finding their optimum 

 life-conditions in depths less than 250 m. With regard 

 to their occurrence beyond this depth we may distinguish 

 the stenothermal species, which only occasionally penetrate 

 into the colder waters of the deep sea, from the euryther- 

 mal ones, which seem to be equally at home in the 

 surface layers and in the deeper waters. It is of interest 

 to note that the species with the widest horizontal distribu- 

 tion have also the greatest vertical range. Besides Clio 



Fig. 37. Hydrographical conditions at 900 metres? The -stations where Peraclis diversa was taken 



are indicated by crosses. 



Atlantic, and their occurrence in the open ocean may be 

 dependent upon the existence of currents carrying them 

 along. Like the Sargasso weed, the species of Limacina 

 inhabiting the western part of the ocean would in that 

 case be considered as belonging to the American coast- 

 fauna, corresponding to the African coast-fauna, but the 

 currents prevent the African fauna from spreading into 

 the neighbouring eastern part of the ocean. 



In the present state of our knowledge we can only 

 raise the question, leaving the answer to future investiga- 

 tions into the biology of these small species of Limacina. 



Vertical distribution. In table III, p. 48, the collec- 

 tion of thecosomatous pteropods from the "Michael Sars" 

 ekspedition is arranged according to the occurence of 

 each species at different depths. 



pyramidata and C. cuspidata we find here also Styliola 

 subula, a warm-water species which however, according 

 to Meisenheimer avoids the warmest waters on both sides 

 of the equator. 



Besides all these surface forms there are three species 

 whose vertical distribution is essentially different. These 

 species, which seem to find their optimum life-conditions 

 in the deeper layers of the ocean (from 500 to 1000 m. 

 or more), are Peraclis diversa, Limacina helicoides and 

 Clio falcata. Their occurrence at these depths and their 

 total absence at the surface is clearly shown in table III, 

 which together with table I (p. 46—47) will give an 

 idea of their distribution, which may now be considered 

 in relation to the hydrographical conditions in the ocean 

 at the time of the expedition. 



