i893. DISTRIBUTION OF MARINE ANIMALS. 95 



tent observers to be relatively rare animals, we might not be 

 astonished to find that the Trachymedusae proper, with the families 

 Trachynemidse, Aglauridae, and Geryonidae, 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 

 tlie 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 naturaUsts. 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 

 Aglauridae were the chief constituents in the northern part, the 

 Trachynemidae in the middle, and the Geryonidae in the equatorial 

 part of the course followed by the ship ; we find Aglauridae and 

 Trachynemidae in the equatorial district too, but in much inferior 

 quantity and variety of forms, so that we are fully entitled to look 

 upon the Geryonidae as inhabitants of the warmer oceans. It is in 

 correspondence with this fact that Geryonidae 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 Trachynemidae 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 Hmit, Florida and the Gulf Stream. Haeckel has already 

 recorded as a strange fact that, in spite of so many explorations, no 

 Trachynemidae 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 aberrant forms have 

 been found in the northern part of the course of the expedition, which 

 probably come from greater depths, and Avhich 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, Solmavis mnltilohata, and Homoionevia (n. gen.), all 



