Dr. W. Marshall on new Siliceous Sponges. 405 
Special Part. 
POTAMOLEPIS, n. gen. 
Monactinellid siliceous sponges of the fresh water of great 
brittleness, with curved, obtuse, smooth spicules, which (when 
dry) are closely cemented together by a small quantity of 
organic substance. No gemmules [statoblasts]. 
1. Potamolepis Leubnitzie, n. sp. 
Forming crusts of 1-1°5 millim. thick, finely porous, of 
yellowish-white colour and silky lustre, exactly of the appear- 
ance of very under-baked wafers. ‘The surface presents a few 
crateriform elevations of 0:25-0-40 millim. in height, standing 
in not very distinct rows upon faint undulations, which divide 
dichotomously in both directions, and frequently disappear, and 
in which a certain parallelism is unmistakable. At the sum- 
mit of each elevation there is an osculum of irregular elongate 
ovate (length to breadth as 1 to 0°5 millim.) and sometimes 
elongate triangular or pentagonal form, separated from each 
other by from 1 to 4 millim., but usually by 2 millim. The 
mouths, which are usually furnished not with smooth but with 
finely-notched margins, lead into very shallow (0°5 millim. 
deep) gastric spaces, which immediately break up into 
several canals ; in the angular mouths the opening of a canal 
of this kind is in general placed im each angle, ‘The canals 
run horizontally, branch dichotomously, and communicate 
frequently with the canal-systems of neighbouring oscula. 
The very numerous and closely-placed incurrent apertures are 
round and about O°1 millim. in diameter; between them are 
situated many much finer apertures, which, in the fresh state, 
are probably covered by ectoderm. 
The three specimens before me formed a thin crust coating 
flat fragments of stone. Many parts of the latter, especially 
where other animals (apparently dipterous larve forming 
adherent cases of sand) had seated themselves, were not over- 
grown. 
2. Potamolepis chartaria, n. sp. 
Oral cone isolated, with round entire margins 0-5-1 
millim. in diameter ; incurrent orifices not numerous, O°1 mil- 
lim. in breadth. Surface like blotting-paper, with a dermal 
skeleton composed of very delicate, felted straight acerates 
attaining a length of 0°08 millim. Colour in the dry state 
chocolate-brown. 
This species much resembles the preceding in habit and the 
