GENUS ORBITOLITES : GENERAL PLAN; SIMPLE TYPE. 197 



cell (a) of a somewhat pyriform shape, as distinct from the space (b, b) which sur- 

 rounds it. The meaning of this feature is at once made apparent, by reference to 

 the disposition of the sarcode which occupies the cavity of the nucleus ; for we then 

 see (Plate IV. fig. 5) that the large central pear-shaped cell is occupied by a mass (a) 

 of corresponding shape, from the small extremity of which a peduncular process 

 extends, that dilates again into a still larger mass (b, b) completely surrounding that 

 from which it springs; the former may be conveniently designated as the 'central,' 

 the latter as the ' circumambient ' segment. In a vertical section of the disk, passing 

 through the centre, such as that seen in Plate V. fig. 4, the nucleus seems to present 

 three chambers ; but this is simply due to the fact that such a section will traverse 

 the circumambient cell twice, that is, will cut it through on both sides of the central 

 cell. In the section represented in Plate V. fig. 9, there are but two central cham- 

 bers ; in this case the plane of division seems to have traversed the nucleus just where 

 the neck of the central cell touches its margin, so that the circumambient cell is only 

 on one side of it. If, on the other hand, the plane of division should happen not to 

 pass through the central cell at all, so as to traverse the circumambient cell alone, a 

 single broad cavity will present itself in the vertical section, as shown in Plate V. 

 fig. 7- Frequently, however, it happens that the circumambient segment is partially 

 subdivided on one side by an interposed partition (Plate VII. fig. 4) ; and then a ver- 

 tical section will show four chambers, as is seen in Plate V. fig. 10, the central seg- 

 ment having a single portion of the circumambient segment on one side of it, and a 

 double portion on the other. Some remarkable varieties in the size of the nucleus, 

 and in the mode of its connexion with the surrounding parts, will be noticed here- 

 after (f ^[ 44-46, and 54, 55). 



16. In describing the structure of the Concentric Zones which successively sur- 

 round the nucleus, it will be requisite to make a distinction between the simple and 

 complex types according to which the Orbitolite-disks may be generated; the former 

 being characterized by the existence of only one layer or ' floor ' of segments, the 

 latter by the presence of two or more such layers. For although, as will subsequently 

 appear, I can show, by a series of transitional gradations between these two types of 

 structure, and by their occasional coexistence in the same individual, that they are 

 not to be held to characterize distinct genera (as Professor EHRENBERG supposed), or 

 even distinct species (as Professor WILLIAMSON has urged with more apparent reason), 

 yet, when most characteristically displayed, they differ so much from one another, 

 and each is so remarkably distinguished by features of its own, that it seems more 

 advantageous to describe them separately in the first instance, and then to discuss 

 their relationship to each other. 



17. The Simple type is found to prevail in those minute Orbitolite-disks, which 

 occur in greater or less abundance in sands and dredgings from almost every part 

 of the globe, but which are particularly numerous in those of the Philippine shores. 

 Their ordinary diameter is about '05 of an inch, and they usually contain from ten 



MDCCCLVI. 2 D 



