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. "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 tho 
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. In 
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 protoplasmie 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 protoplasmie 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 water." 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. Тһе process of 
segregation by which the solution is concentrated into nodules, taking 
every vestige of spienle out of the surrounding chalk, needs additional 
explanation. On this subject I quote from Professor Sollas.? In this, ho 
1 Fossil Sponges from Upper Chalk, p. 65, Pl. V. Figs. 17-19, 
2 Quarterly Jour. Geol. Soc., 1880, Vol. XXXVI. p. 88. 
3 Ann. Mag. Nat. Hist., Ser. 5, 1880, Vol. VI. pp. 441, 442. 
