1893. DISTRIBUTION OF MARINE ANIMALS. 95 
tent observers to be relatively rare animals, we might not be 
astonished to find that the Trachymedusz proper, with the families 
Trachynemide, Aglauride, and Geryonide, form the chief contents 
of the Medusan hauls. However, the members of these three 
families participate in the composition of the hauls in a very different 
manner in the different regions, as well in quantity as in species. 
First of all we notice that the common law on the mainland, that 
the number of the species constituting a fauna increases towards the 
_ Equator, obtains also in the sea. In the North Atlantic very often 
one single species, sometimes in an enormous number of individuals, 
formed the contents of the Medusan haul, while near the Equator the 
net always brought different species of Craspedota, sometimes five or 
more, to the eyes of the naturalists. Besides this general law we 
notice that we can distinguish limits of distribution, not only for 
species, but also for whole families; of course, these are not sharp 
dividing lines, such as the currents furnish for some species, but one 
can speak of districts where this or that family is predominant. The 
Aglauride were the chief constituents in the northern part, the 
Trachynemide in the middle, and the Geryonide in the equatorial 
part of the course followed by the ship; we find Aglauride and 
Trachynemide in the equatorial district too, but in much inferior 
‘quantity and variety of forms, so that we are fully entitled to look 
upon the Geryonide as inhabitants of the warmer oceans. It is in 
correspondence with this fact that Geryonide have not been found 
hitherto in the North Sea, and that only one example is known from 
the Atlantic coast of England, whereas they are abundant in the 
Mediterranean, and are well known to the investigator of embryology 
there. 
The Trachynemide cannot be called a tropical and subtropical 
family with as much right, since their abundance in these districts is 
not so great ; however, they do not seem to pass beyond a certain 
northern limit, Florida:and the Gulf Stream. Haeckel has already 
recorded as a strange fact that, in spite of so many explorations, no 
Trachynemidz have been seen in the North Atlantic. We may con- 
firm these statements by the results of the expedition for the typical 
relatives of this family; however, three somewhat abervant forms have 
been found in the northern part of the course of the expedition, which 
probably come from greater depths, and which occur nowhere else, 
and claim special morphological interest. 
A picture of the distribution of whole families can only be a 
rough one, and to get a more precise conception of the fauna we have 
to consider the single species. Thus we are able to divide the ocean 
traversed by the expedition into different regions, with their charac- 
teristic or “‘leading forms.” First, we can distinguish a northern 
district, beginning at the Scottish coast, and having the Gulf Stream 
and the Azores as its southern limits. The characteristic forms here 
are Aglantha digitalis, Solmaris multilobata, and Homotonema (n. gen.), all 
