330 
REVIEWS, 
“ prolonged backwards along the two margins that the chambers have 
“ a sagittate form, the aperture being at the point of each,”—and 
the ‘ dentaline,’ in which the shell is no longer rectilineal, but curved, 
though connected with the absolutely straight Nodosarwa by imper¬ 
ceptible gradations. In these and other cases the form of the aper¬ 
ture alters along with that of the shell and, from being circular, 
becomes oval, crescentic, reniform, or even triangular, while in the 
so-called Himulina (one of the dentaline modifications of Nodosarina ) 
it appears as “ an elongated fissure, extending through a large “ part 
of the convex side of the anterior wall of the chamber.” 
Much more frequent is the spiral # mode of growth, which is 
‘ nautiloid’ (equilateral), when all the whorls lie in the same plane ; 
‘ turbinoid’ (inequilateral), in a series of planes. As the successive 
chambers (and whorls) increase or remain nearly equal in size, so 
does the resulting spiral, whether nautiloid or turbinoid, also vary. 
Again, its earlier whorls may, to a greater or less extent, be invested 
by those which succeed, and in the nautiloid forms especially the 
first-formed whorls are thus often concealed. “ How, the degree of 
“ this investment is determined by the degree in which the successive 
“ segments of the sarcode-body of the animal send out lateral lobes 
“ that extend themselves over the previously formed portion of the 
“ shell; and this is manifested in the shell by the development of 
“ what may be termed the alar prolongations of the chambers which 
“ are the portions formed to include those lobes.” The nautiloid spiral 
is well represented by Folystomella; the turbinoid, by Hot alia, the 
best type, perhaps, of the entire group of Poraminifera. In Miliola 
we have a curious modification of the nautiloid spire; the shell being 
elongated in the direction of one of its diameters, the opposite 
extremities of this indicating the limits of the two chambers, of 
which each whorl is made up. So, on the other hand, we may regard 
as modifications of the turbinoid mode of growth such shells as those 
of Textularia and JBulimina. In the former, the number of cham¬ 
bers in each plane is so small that the shell appears to consist of a 
double series of segments, alternating on either side of a straight 
axis. Bulimina (in its more usual form at least) carries the same 
modification still further, for here the spire becomes uniserial. 
The cyclical mode of growth is best exhibited by Orbitolites and 
Cycloclypeus, and has been made intelligible to all interested in the 
study of the Foraminifera since the appearance of the first series of 
Dr. Carpenter’s 4 Researches ’ on these animals, published in the 
Philosophical Transactions for 1856. In this Memoir the structure 
of Orbitolites was minutely described, the curious variations to which 
this type is subject being, likewise, noticed at considerable length. 
And in the second series of the same essays the more complex organi¬ 
zation of the much rarer Cycloclypeus was also made clear. In both 
these discoidal shells the chambers form a succession of concentric 
zones, each completely surrounding its predecessor, with which it 
communicates by its series of marginal apertures. The chambers (as 
* Or helical. IIelicoidea of Scliultze. 
