CARPENTER ON THE STUDY OF THE FORAMINIFERA. 
329 
Among the Monothalamia proper the shell is either spheroidal 
( Orbulina ), ovoid ( Ovulites ), or flask-shaped ( Lagena ). In Squamu- 
lina, a non-perforate form, it has the figure “ of an irregular plano- 
“ convex lens, being usually flat, or nearly so, on its attached side 
44 (which accommodates itself to the surface whereon it grows), and 
44 convex on its free side, on some part of which—usually about half 
“ way between the centre and the periphery—is a wide orifice, from 
44 which the pseudopodia issue.” In Lagena the neck of the flask 
presents a small circular aperture, which is commonly everted and 
furnished with radiating notches. The principal orifice of Orbulina 
is scarcely distinguishable from the larger pores. Ovulites , on the 
other hand, has two conspicuous apertures, one at each end of its 
shell. 
The form of the shell among the true Polythalamia is determined, 
in part, by that of its several segments, chiefly, however, by the 
mode in which these are added to one another. The segments may, 
likewise, differ as to size, those first formed being usually the 
smaller. Three principal directions of growth have been observed : 
—the straight , the spiral , and the cyclical. Between these, however, 
intermediate varieties occur, and almost every type exhibits its own 
minor modifications. Irregular, or 4 acervuline’ * growths also exist, 
in which the typical mode of increase, whatever it may be, is marked 
by the influence of secondary causes. And in such Toraminifera as 
are attached, the surface on which the shell rests may be brought 
more or less into play as a modifying agent. 
The straightf mode of growth is not a very common one, and is 
exhibited in perfection only by two generic forms, closely related to 
Lagena. In one of these, Orthocerina , no external constrictions are 
manifest between the successive segments of the shell. In Nodo- 
sarina such constrictions exist, and the “ segments sometimes ex- 
“ hibit a progressive increase, so that the entire shell has somewhat of 
44 a conical form; whilst in other cases each segment has the same 
“ diameter as the preceding, so that the general form of the shell is 
“ cylindrical; and it occasionally happens that the middle segment is 
44 larger than those which precede and follow it, or even that the 
“ primary segment may be the largest, and that the size of the suc- 
44 ceeding segments progressively diminishes. The primordial segment 
44 is often furnished with a more or less prolonged mucroL The 
same form presents itself under some interesting varieties, the two 
most remarkable of these being the 4 frondicularian,’— 44 of which the 
44 shell is extremely compressed, and of which the segments are so 
* Dr. Carpenter (p. 209) has shown that the Acervulina of Scliultze, who 
established for the reception of this genus a special primary group (Sokoibka), “ is 
nothing else than a Planorbulina, which attaches itself to the stem of a Zoophyte 
or some other small rounded body, and of which the segments, instead of spreading 
discoidaliy over an expanded surface, cluster together in a mass, with more or less 
of regularity of arrangement.” 
f Or rectilineal. The Polythalamia presenting this mode of growth correspond 
to the group Bhabdoidea of Schultze. 
