16 Transactions of the Society. 



level with the contonr-surface of the test, and discoverable only 

 with difficulty, can be traced through successive stages until it 

 becomes perfected into the produced nipple-shaped orifice of the 

 adult organism. 



Psammosplimra fusca (Schulze). 



This species has a much wider distribution, both as regards the 

 depth of its occurrence and the condition and nature of the sea- 

 bottom, than Saccammincc. While Saccammina appears to be 

 extremely local in its distribution, and to be confined to dredgings 

 in which there is an abundance of mud and little current action, 

 Fsammosj^haera is to be found in much shallower waters, and even 

 in shell-sands and gravelly deposits where little or no mud is 

 present. 



Psammosphsera fusca is essentially protean, both as to its 

 external form and as to its method or plan of construction, but 

 such variability is, in our opinion, largely due to the nature of its 

 surroundings. When occurring in the deeper and muddier de- 

 posits, co-existent with Sacca7nmina, Psammosphsera fusca follows 

 practically the same lines of development and growth, differing 

 only from Saccammina in its more irregular method of construc- 

 tion, in the greater quantity of ferruginous cement which is 

 habitually employed, and in the characteristic and essential ab- 

 sence of any aperture. 



In shallower waters where it is more likely to be subjected to 

 wave- or current-disturbances, and where Saccammina dies out, 

 Psammosphdera fusca continues to flourish, but shows a marked 

 tendency to adopt a sessile form, adhering to large sand-grains, 

 small pebbles, and molluscan fragments. The nature of the shell- 

 wall may continue roughly and entirely arenaceous, or it may pass 

 by insensible gradations into a neat hemispherical test, in which 

 the embedded sand-grains form a smaller and smaller proportion 

 of the whole, until we arrive at a stage found in many of the 

 shallower dredgings off the east coast of Scotland, in which the 

 larger sand-grains have disappeared by degrees, leaving a test in 

 which the hemisphere is entirely composed of the finest sand-grains 

 and a ferruginous cement. Such specimens are distinguishable 



EXPLANATION OF PLATE III. 



Figs. 1-3. — Crithionina mamilla A. Goes. Early stages attached to fragments of 



Molluscan shell, 

 >, 4. Ditto. A more advanced stage attached to Saccammina sphaerica 



M. Sars. 

 ,, 5-7. Ditto. Adult form attached to Astrorhiza arenaria Norman, 



fig. 5, and to Saccammina sphxrica M. Sars, figs. 6, 7. 



All figures magnified 35 diam. 



