GENUS POLTSTOMELLA : EXTERNAL CHARACTERS. 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, , b) is 

 distinguished from 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 

 sufficiently 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. 1, J), 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. 1, a), 

 two such rows usually intervening between each septal band and that which precedes or 

 follows it (Plate XVIII. fig. 1, hh, h'h'). In the older portion of the last formed whorl, 

 it is sometimes to be observed that these punctations with their surrounding dimples 



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