MERRILL : FOSSIL SPONGE SPICULES. 7 



zone spicules, and are therefore present in greater numbers. These will 

 be discussed in detail later. 



3. Mollusks. Small fragments of what I thought nacreous and pris- 

 matic tissue of shells of mollusks were found in two of the nodules. 

 The condition of their preservation was not sufficiently perfect, however, 

 to make their identification absolutely certain. They are replaced by 

 amorphous silica, which in taking their structure has been turned a 

 bright transparent yellow. 



4. Fishes. Several organisms having the general form of fish scales 

 were found, but no attempt has been made to identify them with modern 

 or fossil fishes. They are of a bright transparent yellow, and have the 

 appearance of organic silica. Their surfaces are perfectly smooth, and 

 the outline entire and complete. In the polarizer they show no effect 

 different from that produced by the amorphous silica surrounding them. 

 I concluded, therefore, that they are amorphous silica similar to the 

 shell fragments previously described. 



Condition of Preservation of the Sponge Spicules. 



Spicules are found in these flints in all stages of preservation. But 

 few were found perfect except the globo-stellates and similar spicules of 

 the dermal layer. Occasionally, areas in the slide were found containing 

 numerous faint tracings of spicules which are so merged together that 

 they cannot be separately traced. More often, however, they are 

 separately embedded in a mass of amorphous silica. Under the high 

 power of the microscope, except in cases where the spicules seem to end 

 indefinitely in the surrounding silica, the outline is perfect, but more or 

 less ragged, owing to the irregularities of replacement. The canal often 

 shows a separate crystallization from the body of the spicule, and is gen- 

 erally smooth in outline. The canal differs greatly in size in the different 

 specimens, and in some occupies the entire body except a small ring of 

 crystalline silica on the outside. Apparently the replacement began 

 along the axial canal at the same time as it did on the surface, and in 

 some cases, as in Figure 5, it seems that most of the spicule has been 

 replaced from the inner side. 



In this spicule, and also in others, the canal seems to be replaced with 

 a dark opalescent silica mixed with grains of siliceous sand. Replace- 

 ments with sandy material are common in the English flints. In other 

 spicules the canal is perfectly transparent and hyaline in appearance, 

 and in a few cases the canal is not continuous (see Figure 1). In 



