GENUS POLYSTOMELLA. 279 



backwards for a short distance from both the outer or lateral margins of each segment of the 

 sarcode-body, and then terminate abruptly. From the neighbourhood of the inner arch of each 

 segment, on the other hand, there proceeds a series of threads of sarcode (fig. 6, c) much 

 slenderer than tlie " retral processes " just described, which unite each segment to the two 

 contiguous segments before and behind, passing through the row of pores already mentioned 

 as visible along the inner margin of the septum. The substance forming the spiral lamina is 

 finely tubular ; but no such tubuh arc discernible in the septa, which exhibit only the > -shaped 

 rows of septal pores (fig. 5, r, c), whose number progressively increases with the dimensions 

 of the septal plane, and indications of interseptal canals, d, d, which seem to resemble those of 

 Operculina in their derivation from a pair of spiral canals whose transverse sections are seen 

 at e, c The solid umbilical deposit, /, V , is traversed through its whole thickness by straight 

 parallel canals proceeding directly towards the external surface, where their terminations 

 form the punctations already mentioned. 



473. PoLYSTOMELLA CRATicuLATA : Evtcnuil CJiaracters. — The Australian examples 

 of P. craticulafa (Plate XVI, figs. 1, 2, a, b), are remarkable not only for their comparatively 

 large dimensions (the diameter of some of the specimens in my possession exceedino- one- 

 sixth of an inch), but for the considerable proportion of their two lateral surfaces occupied by 

 that solid calcareous nucleus which is confined in other forms 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 maro-in the 

 chambers of the last-formed whorl (as is made evident by vertical sections, fio-. 3 / / /' /') 

 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 umbilical nucleus, however, to be suflaciently 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 focussed down to their surface and a strong light 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 (figs. 1, 3), the thickness of each whorl (/. e. the 

 distance between its two lateral surfaces) being greater than that of the preceding, and the 

 later whorls not extending themselves over those previously formed. The septa are marked 

 externally 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. The surface of the central nucleus is marked at pretty regular intervals with 

 minute punctations (fig. 2, a), each of which occupies the centre of a little dimple or depres- 

 sion ; 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. 2, b), two such 

 rows usually intervening between each septal band and that which precedes or follows it 

 (fig. 1, hh, h'Ji'). In the older portion of the last-formed whorl, it is sometimes to be observed 

 that these punctations with their surrounding dimples constitute the only interruption to the 



