REPORT ON THE TETRACTINELLIDA, 
397 
A 
I u m isr 
I n mr n? 
Fiq. 3.— Curves illustrating the bathymetri- 
cal distribution of the Tetractinellida as 
deduced from the Challenger observation. 
A. Bathymetrical curve of the Tetrao- 
tinellida compared with that of the other 
groups of Sponges and with that of all 
the groups taken together. The dotted 
lines indicated by letters with a dash 
refer to the distribution of the number 
of successful Stations, the continuous 
lines to the frequency of the occurrence 
of species ; a, a', curves of the Sponges 
taken all together ; h, V, of the Monax- 
onida ; c, d, of the Hexactinellida ; d, d', 
of the Tetractinellida ; e, d, of the Cal- 
canea ; /, /,' of the Ceratosa. B. Bathy- 
metrical curves of the Tetractinellida 
and of its two orders — Choristida and 
Lithistida. T. Tetractinellida ; C. 
Choristida ; L. Lithistida ; I., depth 
from 0 to 50 fathoms ; II., from 51 to 
200 ; III., from 201 to 1000 ; IV., from 
1001 to greater depths. 
