GENUS Fl SILINA. 305 



p. 380), the Borrlis cou.'slrivld, Ehr., has been supposed to be differentiated by a constriction in 

 the middle of its length; but this is onlj' an occasional modification. When the shell can be 

 sufficiently freed from its matrix for the characters of its surface to be distinguished, this is seen 

 (Plate XII, fig. 27j to be marked out by ])arallel septal bands running in the direction of the 

 axis, into a succession of segments of nearly uniform breadth ; but these segments are not 

 crossed on the surface, as they arc in , /Ave////'/, by regular and distinct secondary furrows, 

 though there is sometimes an external indication of that subdivision of the principal chambers 

 which will be shown to exist in the interior. The form of the septal plane (fig. 27, <\ e) very 

 closely corresponds with that of Aheolinn, as it is comparatively narrow in the centre of its 

 length, and spreads out widely at its extremities ; but the nature of the aperture differs com- 

 pletely. For instead of a single or multiple series of pores extending along its entire 

 length, we here find only a single slit-like fissure (fig. 27, «) limited to its middle region ; the 

 limitation of the communication between the successive elongated segments, and between the 

 last of these segments and the exterior, thus contrasting very strongly with the freedom which 

 we have seen to prevail in the segmental communications of Alceolina, whilst it closely cor- 

 responds vrith that which is characteristic of the Nummuline series. 



512. Interna/. S/nicfure. — In this, as in most other fossils of the Carboniferous Lime- 

 stone, the ultimate texture of the shell has been so far altered by molecular change as to 

 make it impossilile to speak with certainty as to its character. But in several of the sections 

 which I have made, I find an appearance (fig. 26) indicative of a porosity resembling that 

 of the Nummuline and Rotaline shells ; this appearance, so far as I can judge, having been 

 produced by closely set, parallel tubuli, intermediate in diameter between the fine tubuli 

 characteristic of the former group and the coarse pores usually seen in the latter. In the 

 very numerous sections which I have made of Alvcoliua and other porcellanous Foraminifera, 

 fossil as well as recent, I have never seen any appearance which at all corresponds to this ; 

 and I cannot but think it much more probable that it indicates a definite structure which has 

 been in great degree altered by molecular change, than suppose that the homogeneous 

 texture of a porcellanous shell has given place to one in which differentiation of some kind is 

 so distinctly marked. The general structure of the interior of the most developed forms of 

 this type will perhaps be best understood from the " cast" figured by Prof. Ehrenberg (xlii) 

 from one of its simpler forms ; for we see in this (fig. 24), an arrangement very like that which 

 the segments of a simple Nummuline shell would present if their alar prolongations were to 

 extend themselves on either side in a line with the body of each segment, instead of folding-in 

 towards each other as they do in discoidal shells. The difference between this and the 

 ordinary more complex type, of which sections are represented in figs. 25 — 29, consists essen- 

 tially in this, that in the latter each of the elongated alar prolongations is broken up into a 

 necklace-like series of sub-segments connected by a longitudinal stolon. In the transverse 

 section taken across the median portion of the shell, represented in fig. 25, we see the spiral 

 succession of the principal chambers, which are divided from each other by septa formed by 

 the inflection of the external wall ; these septa, which are composed of a single lamella, do not 

 reach the surface of the penultimate whorl, but leave a .fissured passage («), through which 

 the chambers communicate with each other (as is better seen at ", a, fig. 26), just as 

 in the ordinary shells of the Nummuline type. The same transverse sections show the 



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