20 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



imperfect state, has led me to doubt whether it can be referred to this 

 position. Its color and appearance in polarized light indicate that 

 it is colloidal silica, and its arms seem to cross at right angles. In form 

 it may be considered somewhat similar to genus Leptophragma, Zitt., 

 several forms of which are figured by Hinde. 1 The size of the entire 

 figure is 0.0615 mm. by 0.046 mm. It is placed here doubtfully, there- 

 fore, because of similarity of form, but as these sponges may be supposed 

 to have died in all stages of growth, it is reasonable to suppose that the 

 size would be subject to much variation. The one figured was- the only 

 one found. 



Formation of the Nodules. 



The question of the formation of the flint nodules has received con- 

 siderable attention in the past, and the problem may not yet be alto- 

 gether solved. Although I do not expect to throw any light on the 

 subject by this brief study, yet I hope to use the observations I have 

 made as illustrations in the discussion of the conclusions of others, lu 

 order to get the question fairly before us, I quote from Dr. Wallich : 

 " The stratification of the flints is due to the fact that nearly the whole 

 of the silex derived from the sponges on the one hand, and the continual 

 subsidence of minute dead siliceous organisms on the other, is retained 

 in the general protoplasmic layer which I have shown maintains its 

 position on the immediate surface of the calcareous deposit and gradually 

 dissolves the silex. This layer in virtue of its inferior specific gravity 

 rises with every increase in the thickness of the deposit, until at last the 

 supersaturation of the protoplasmic masses with silex takes place, and 

 the first step towards the consolidation of the flint is accomplished, — 

 the continuity of sponge life and of the various other forms which ten- 

 ant the calcareous bottom being secured through the oozy spaces which 

 separate the sponge beds, and thus admit of both adult and larval forms 

 having free access to the overlying stratum of wafer." 2 



While this refers primarily to the formation of strata of flint rather 

 than nodules, yet the solution and distribution of the solution of the 

 siliceous organisms must be the same in both cases. The process of 

 segregation by which the solution is concentrated into nodules, taking 

 every vestige of spicule out of the surrounding chalk, needs additional 

 explanation. On this subject I quote from Professor Xollas. 8 In this, he 



1 Fossil Sponges from Upper Chalk, p. 65, I'l V. Figs. 17-19. 



2 Quarterly .Jour Geol. Soc.,1880, Vol. XXXVI p 



3 Ann. Mag, Nat Hi.st., Scr. 5, 1S80, Vol. VI. i>p. Ill, II- 



