162 FAMILY LAGENIDA. 



an apparently homogeneous shell-substance. In those forms which have the segments closely 



applied one to the other, the outer wall of each chamber partly invests the outer wall of the 



preceding, and the invested portion serves as the septum between the two 



Fig. XXX. ^pig._ XXX). This septum has the ordinary structure of the outer wall, and 



««, is not diiferentiated from it by being non-tubular, as is ordinarily the case 



in other shells of the vitreous series ; a point, as we shall presently see, 



of some consequence in regard to the position of the aperture in Cristellaria. 



The apertures a, «,V/,"«,' between the successive segments correspond precisely 



in form and position with that of the last segment «* which is seen externally. 



247. Cristellaria : TSxternal Characters. — The typical form of this 

 organism may be considered as a nautiloid spiral, each wliorl of which has 

 its chambers extended by alar prolongations so as to reach the centre, 

 and thus entirely to enclose the pi'eceding whorl. The number of chambers 

 in each whorl is much smaller than in most other nautiloid Foraminifera, being 

 seldom more than eight or nine, and often less. The division of the chambers 

 is always strongly marked externally by septal bands, which are sometimes elevated into 

 continuous ridges, sometimes into rows of tubercles, whilst sometimes on the contrary they 

 are depressed ; but they are always differentiated by their peculiar appearance from the 

 ordinary surface of the shell. The direction of these septal bands is somewhat oblique to 

 the axis of the spire ; and they are generally convex anteriorly (ex, figs. 52 — 54). They meet in 

 the umbilicus, which is usually raised into an elevated tubercle. The margin of the shell 

 often rises into a sort of keel or carina, which may even be extended into a cullro or knife- 

 blade projection ; and sometimes this gives off a variable number of radiating prolongations 

 (Plate XII, fig. 3), as in the form distinguished as C. calcar. The surface of the segments 

 between the septal bands is sometimes regularly marked by ridges passing in the direction 

 of the ajcis of growth, so as precisely to correspond with those of Nodosaria, as in the 

 Mobulina ariminensis of D'Orbigny ; whilst it is sometimes studded with little spinous tuber- 

 cles, as in the R. eckinata of D'Orbigny; and the one condition may give place to the otiier 

 in different segments of the same shell, as is shown in Plate XII, fig. 3. The form of the 

 septal plane is sagittate (Plate XII, fig. 3) ; and the aperture, which exactly resembles that of 

 Nodosaria, is situated at its extreme point, so as to be at the outer margin of the convolution. 

 When the shell is looked at laterally (Plate XII, fig. 3), however, it is seen that the aperture 

 is really situated nearly in the centre of the anterior wall of the last segment, as in Nodosaria. 

 — The diameter attained by the fossil Cristellarice of the Sub-Apennine Tertiaries occasionally 

 exceeds l-5th of an inch ; the recent examples of this type, however, are usually far 

 smaller, the diameter of the British specimens not usually exceeding 1-1 5th of an inch, and 

 being frequently much less. 



'' 248. Internal Structure. — The te.xture of this form of shell precisely corresponds with that 

 of Nodosaria; being very finely tubular, not only in the lateral walls, but also in the septal 

 divisions between the chambers, which are here strictly continuations of the lateral walls over 

 the anterior face of the chambers, instead of being completely differentiated from them as in 

 the higher types of the vitreous series. That the real position of the aperture is nearly in the 

 centre of the anterior wall of the chamber, is made very apparent by a careful examination of 



