GENUS POLTSTOMELLA : — EXTEENAU CHAEACTEES. 
539 
have been thus infiltrated. These casts represent not merely the segments of the 
sarcode-body with their connecting stolons, but also those prolongations of the body 
which occupied the canal-system ; and as they preserve with the greatest exactitude the 
natural forms and relative positions of these parts, they really afford more precise and 
satisfactory information than that which could have been derived from an examination 
of the sarcode-body of the animal itself, since its softness and friability are such as 
greatly to interfere with the due appreciation of its characters, when it is deprived of 
the support afforded by the shell. 
179. External Characters . — This type of Polystomella (Plate XVII. fig. 1, a,b) is 
distinguished firom the others already noticed, not only by its comparatively large 
dimensions, — the diameter of some of the specimens in my possession exceeding one- 
sixth of an inch, — ^but by the considerable proportion of its two lateral surfaces occu- 
pied by that solid calcareous nucleus which is confined in other species to the umbilical 
region. The diameter of this nucleus is usually about three-fifths of the whole diameter 
of the specimen ; so that it covers and conceals all the earlier convolutions, meeting at 
its outer margin the chambers of the last formed whorl (as is made evident by vertical 
sections, Plate XVII. fig. 2), which are consequently the only chambers that show 
themselves externally, although the last formed whorl does not itself extend far over 
the preceding. I have not unfrequently found this central nucleus, however, to be 
sufliciently transparent (after its surface has been cleaned by a short immersion in 
dilute acid) to allow of the inner convolutions being discerned through it, when the 
microscope is focused down to their surface, and a strong light is directed upon 
this; and it then becomes obvious that, if the solid nucleus were removed, the form 
of the shell would be bi-concave instead of bi-convex, the thickness of each whorl 
{i. e. the distance between its two lateral surfaces) being greater than that of the pre- 
ceding, and the later whorls not extending themselves over those previously formed. 
The septa are marked externally (as in most other Foraminifera of the nautiloid type) 
by bands which indicate their junction with the outer walls of the chambers: these 
bands are meridional (so to speak) in their direction, extending from the margin of 
the nucleus on one side to that of the nucleus on the other side ; they are not usually 
(in adult specimens at least) either elevated above or depressed below the surface of 
the walls of the chambers on either side of them ; but they are distinguished by their 
difference of texture, their substance being much more transparent and glistening than 
that of which those walls are composed. 
180. The surface of the central nucleus is marked at pretty regular intervals with 
minute punctations (fig. I, Z>), each of which occupies the centre of a little dimple or 
depression ; and rows of similar punctations are very commonly seen to extend from the 
nucleus on either side, in a direction corresponding to that of the septal bands (fig. I, «), 
two such rows usually intervening between each septal band and that which precedes or 
follows it (Plate XVIII. fig. I, M, h!h!). In the older portion of the last formed whorl, 
it is sometimes to be observed that these punctations with their surrounding dimples 
4 B 2 
