THE TANGANYIKA PROBLEM. 297 
us with the first instance of an incrusting gymnoleamatous 
fresh-water form. So far as I have been able to compare 
it with the known marine forms of the gymnoleamata, this 
new species from Tanganyika does not exhibit a close 
identity with any of them. The colony in its general 
character, and in some special features of the individuals of 
which it is composed, is not at all unlike the marine genus 
Arachnidium , and but for the greater irregularity of the 
ceils and the longer tubes which are associated together in 
the Tanganyika species, I should have been inclined to 
place it in that genus. Still, considering its remarkable 
habitat, these structural distinctions, and also that I have 
not yet had time to completely investigate the matter, I 
think that the method least open to objection is to form a 
new genus for its reception, and I therefore propose the 
generic and specific names of Arachnoidia ray lankesteri , 
leaving a fuller description of the organism for a separate 
memoir. 
