560 DE. CAEPENTEE'S EESEAECHES ON THE FOKAMESTFEBA. 



on the equatorial plane ; and in this manner the increase of the organism in thickness 

 is effected. The growth on the two sides of the equatorial plane, however, is seldom or 

 never symmetrical ; and that of the more convex portion seems continually tending to 

 overpower that of the opposite surface, so that the equatorial plane becomes more or 

 less deeply concavo-convex. I have reason to believe that this inequality is due to the 

 attachment of the flat or subconcave base to the surface of sea-weeds or zoophytes ; in 

 virtue of which it will naturally happen that the free side will grow faster than the other. 

 It is by an excess of this predominating growth that the spheroidal form is acquired, with 

 its deep residual cavity, as just now described. 



216. On more minutely examining the structure of the walls of the chambers and the 

 mode of communication between their cavities, we find ourselves carried back to the 

 lower type of Orbitolit.es and Orbiculina in this particular, that the septa are not double 

 but single, and are formed of a simple homogeneous substance, presenting no vestige 

 whatever of that fine tubulation which characterizes the dense almost ivory-like sub- 

 stance forming the walls of the chambers in the more elevated forms of this group. It 

 will be convenient to speak of the partitions between the chambers as horizontal, when 

 their general direction is parallel to that of the equatorial plane, and as vertical when 

 that direction is perpendicular to it ; their actual directions will of course vary with the 

 curvature of the equatorial plane. The horizontal partitions or floors of the chambers 

 are perforated by rounded apertures (Plate XXI. figs. 2, 3, 4) which closely resemble 

 those of the shell of a Eotalia or a Planorbulina in their size and arrangement ; and 

 these will allow of free communication, by pseudopodial threads of sarcode, between the 

 segments that are lodged in the chambers piled one over the other in a vertical direction. 

 The vertical partitions are much thicker, and are not thus minutely and regularly per- 

 forated ; but they exhibit a small and variable number of large 

 apertures (fig. 4, a, a), that lead into the adjacent cells which lie A 

 in or near the same horizontal plane. I say in or near, because it 

 is seldom if ever the case that the horizontal partitions or floors 

 of two adjacent vertical piles of cells are on the same level ; and, 

 in fact, the typical arrangement (though frequently departed from) &y_ 



seems to be, that there is an alternation in the levels of the floors 

 of adjacent piles (as shown in the accompanying diagram, based on some parts of fig. 3), 

 and that every chamber in any pile B normally communicates with two chambers in each 

 of its adjacent piles A and c, by one passage above and the other below the floor that 

 divides them. 



217. The relation of this interesting type of structure to that of Planorbulina appears 

 to me so clear that it can scarcely be questioned. For, as in that genus, the first-formed 

 portion of Tinoporus Icevis will evidently consist of a flattened disk, consisting of nume- 

 rous segments which are arranged in one plane, spirally in the centre of the disk, but 

 clustered irregularly towards its circumference, and perforated on both sides with 



B 



