6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
nucleus may or may not be present. It is evident also that different 
nodules may have been formed in different ways, and hence we shall 
confine our conclusions to the limits of our observations. 
4. In the summary we shall discuss the bearing of this study on the 
depth and condition of deposition of the chalk of the surrounding 
formations. 
Organisms found in the Flint. 
Slides taken from the same nodule were found to present great simi- 
larity of kinds of organisms, but great difference in size. А species of 
spicule, for instance, is often found in all stages of development in the 
same slide. Different nodules, however, show great variation in the 
prevailing kinds of organisms and the condition of their preservation. 
On the whole, however, the number of organic remains is few, and the 
massive silica. greatly in excess. When viewed with high power of 
microscope, this massive portion appears often to consist of aggregations 
of minute granules varying in color from brown to almost colorless. 
Dr. Hinde? found the same true in cherts of the Greensand, and speaks 
of the granules as having “circular outlines, though not strictly a 
spherical form." Chalcedonic masses with their characteristic concentric 
structure are common in some of the nodules and rare in others. Many 
traces of organisms, the outlines of which were but dimly shown, were 
not made out; but those determined were of the groups given below. 
1. FORAMINIFERA. These were found in every slide examined, and 
were quite numerous in one of the nodules. The types found were 
principally of the family Globigerina, of which several different species 
were common. 'Textularia were found in most of the slides, though not so 
common as Globigerina, and several other families were found occasion- 
ally. These forms were generally replaced by amorphous silica, the 
outlines remaining in a dark substance which has the appearance of 
organic residue. Globigerina were also found replaced in chalcedonic 
silica, but the outlines and structure of the chalcedonie mass almost 
obliterated the form of the fossil. 
2. SPONGES, In two of the nodules the spicules were of frequent 
occurrence, but not very abundant; but in one nodule of which I had 
two sections, not a distinct trace was found, except occasionally a small 
fragment of the spicule of a Monactinellid shown in Figure 7, Tho mi- 
nute dermal spicules are, generally speaking, better preserved than the 
1 Sponge Remains of Lower and Upper Greensand, Phil. Trans. Royal Society, 
Part IL, 1885, p. 427. 
