RHIZOPODA — FORAMINIFERA. 85 



forming a spiral. Chambers disposed alternately along two or three 

 axes, also non-spiral. 



VI. Agathistega. — Body composed of segments wound round an 

 axis. Chambers formed round a common axis, each investing half 

 the circumference. 



One of the simplest forms of Foraminifer is illustrated by Fig. 17 

 [Orbulina universa), which is a small spherical shell, having a lateral 

 aperture, the interior of which has been occupied by the living 

 sarcode, to which the shell owes its existence. In Dentalina subarcuata 

 the shell (Fig. 18), advances beyond this simple type by a process of 

 linear budding, the first cell being spherical, A\ith an opening through 

 which a second segment is formed, generally a little larger than the 

 first. This new growth is successively followed by others developed 

 in the same way, until the organism attains its maturity, when it ex- 

 hibits a series of cells arranged end on end, in a slightly curved line. 



In the next group the gemmation takes a spiral form, producing 

 the nautilus shape which misled the earlier naturalists. In some 

 cases all the convolutions are visible, as in Opercidina (Fig. 19). In 

 others, the external convolutions conceal those previously formed, as 

 in Nummjilina perforata (Fig. 20), Textularia variabilis (Fig. 22), 

 and Alveolifia ovotdea, d'Orbigny (Fig. 16), the latter of its natural 

 size. 



In the fourth group the shell is spiral, with the chamber equilateral, 

 with a larger and smaller side; the position being alternately reversed, 

 as the segments are multiplied, as in Cassidulina icevigata (Fig. 21). 

 In the succeeding group the new segments are arranged alternately 

 on opposite sides of the central line, as in Textularia variabilis 

 (Fig. 22), thus forming two alternating non-spiral parallel segments, 

 each connected by a single orifice. 



The sixth family differs entirely in appearance and structure from 

 the other Foraminifera. The shells are more apaque than those 

 met with in the other orders, having a resemblance to white por- 

 celain, and present a rich amber-brown hue when viewed by trans- 

 mitted light. They are more or less oblong, each new segment being 

 nearly equal to the entire length of the shell, so that the terminal 

 orifice presents itself alternately at its opposite extremities, some- 

 times in one uniform plane, as in Spiroloculina depressa (Fig. 23), 

 and Rotalia (Fig. 24). At other times each new segment, instead of 

 being exactly opposite each other, is a little on one side. 



This classification of d'Orbigny must now peld to that proposed 

 by Dr. Carpenter in his work before referred to. He divides the order 



