Rhizopods from the Sihkim Himalaya. By. E. Penard. 275 



form, depressed, having the dorsal face rounded and the ventral 

 flat or slightly concave in the centre ; seen from above (fig. 1) the 

 shell has an elliptical, not circular, contour, the greater diameter 

 being to the lesser, usually about as 4 : 3. The shell is formed of little 

 silicious particles, derived from without, always flat and thin, 

 varying in size in different individuals, and always smaller in the 

 vicinity of the mouth. These particles, themselves colourless, are 

 joined by a chitinous cement, which forms a network of nerves, 

 and sometimes extends over the plates, giving the shell a 

 yellowish-brown tint, often inclining to black or to chocolate- 

 brown. The proportion of chitin is greatest on the peristome. 



There are never processes or horns such as are found in the 

 genus Centropyo:is. 



It is on the flat face, which I regard as ventral or inferior, that 

 the buccal slit opens ; parallel to the line of greatest diameter, and 

 at about one-third of the distance from one margin of the shell to 

 the opposite margin, is seen the upper lip, like a narrow band, 

 long, dark-coloured, rather variable in form, but most commonly 

 arched or undulate on its anterior border, the two ends vanishing 

 in points towards the commissures which unite it to the lower 



lip- 



This upper lip, slightly incurved, is formed of minute silicious 



particles, joined by a very abundant chitinous cement, and round 



the whole periphery, as far as the commissures, one sees here and 



there little clear spots, round or elliptical, which are, in fact, 



perforations. 



These, pores, 2 • 5-3yL4 in diameter, with a yellowish border, 

 often raised into an annular edge, are usually about thirty in 

 number. They are not very regularly disposed, but sometimes a 

 kind of symmetry in their arrangement can be detected ; at all 

 events they are never grouped all close together. The pores are 

 lacking on the lower lip, though sometimes, and exceptionally, 

 one or two extend on to the central portion of the inferior face of 

 the shell.* 



To form the lower lip the ventral surface is slightly depressed, 

 forming a shallow basin, composed of small silicious particles, 

 strongly veined with chitinous deposits. Reaching the buccal 

 commissures, where the corners of the upper lip are abruptly bent 

 down to meet it, the free anterior edge of the concavity is continued 

 forward under the upper lip, thus forming a lower lip, arched 

 forward in a curve of regular convexity (fig. 3). 



* If we suppose the animal creeping in the natural position, viz. with the 

 ventral or buccal face downwards, what I have called the upper lip will really be 

 in a lower position, in relation to the substratum, but it seems to me that we 

 must regard as the upper lip that border of the mouth which is external, and 

 further from the centre of the plasma, and as lower lip that border which is 

 internal, or nearer the centre of the plasma. 



