GENUS ALVEOLINA. 101 



more to the length than to the diameter of the shell. In this section the long narrow 

 chamberlets are divided transversely, so that they are merely seen as rows of rounded aper- 

 tures channelled out in the solid substance of the shell ; they correspond in relative situation 

 with the pores of the apertural plane, but are usually of much larger size. Generally about 

 four such rows may be distinguished in each convolution ; but they are far from being 

 regularly disposed, especially towards the two extremities of the axis of growth. Along the 

 outer margin of the last and of each preceding segment, we see a row of apertures much 

 smaller than the rest, and much more closely set together ; these are the transverse sections of 

 tlie superficial layer of chamberlets, the dimensions of which are indicated by the furrowing 

 of the surface, and which are about twice as numerous as those of the rows that lie internally 

 to them. This difference is peculiarly interesting when taken in connection, on the one hand, 

 with that which we have seen to exist between the superficial and the more deeply seated cham- 

 berlets of Fabularia (f 118), and on the other with that which we shall encounter between 

 the superficial and the intermediate" layers of chamberlets in OrbitoUfes (1 16G), the type in which 

 this differentiation seems to acquire its fullest development. The form and arrangement of 

 the chamberlets are better displayed by transverse sections of the shell, such as is represented 

 in fig. 14; in which figure the solid substance is left white, the shallow cavities occupied by 

 the sarcode (here laid open in the direction of their length) are shaded of a half tint, whilst 

 the black spots represent the openings of long galleries that pass continuously from one end 

 of the shell to the other, so as to bring all the chamberlets into free communication with each 

 other laterally. As each whorl is in regard to all essential particulars but a repetition of the 

 rest, it will be sufficient to describe the structure of the outer convolution. This is divided 

 into principal segments, which obviously correspond with the spaces that intervene between the 

 depressed septal bands of the external surface ; the places of those bands being marked, not 

 only by slight inflections of the external outline, but also by prolongations [a, a, a) of the 

 superficial lamella, which are directed inwards so as to contract the mouth of the spire at 

 what may be considered the termination of each formative act. Just within these projections 

 are seen the orifices [b, b, b) of one set of longitudinal channels, whilst nearer the internal side 

 of the convolution are seen the orifices {c,c, c) of another much larger series of longitudinal 

 channels, which are like galleries connecting the contiguous chamberlets of the same floors. 

 Between the orifices b, h, b, and the orifices c, c, c, which respectively correspond to them, 

 there is no division of the principal cavity of the spire into " storeys" of chamberlets ; but, to 

 carry out the analogy, those intervening spaces may be likened to the well-staircases by 

 which a vertical communication is established between the several " storeys" in a large 

 mansion. The principal cavity of each segment is elsewhere separated by the intervention of 

 three concentric laminae of shell {d, d^, d^) into a set of superposed chamberlets {e, e^, e^, e'), 

 which are usually four in number, though in a portion of the convolution that is rather narrower 

 than the rest we see only two lamellse and three chamberlets, whilst in a wider portion of 

 the interior convolution we see four lamellae and five chamberlets, and even this number is 

 frequently exceeded. The thickness of these lamellae is by no means uniform, and the capacity 

 of the chamberlets between which they intervene is subject to variation accordingly. 

 Generally speaking, the lamellae are thickest at their posterior extremities, so that the entrances 

 to the chamberlets from the vertical " wells" are there much narrowed ; in front of these 

 constrictions the cavities of the chamberlets usually open out considerably ; and they become 



