86 JOURNAL OF THE [April, 



alveoli arranged in Brebisson's quadrille seem to have strength- 

 ened ribs between the rows of "dots" — P. balticiim, P. attenuatinn, 

 etc , have them longitudinal and straight. Mr. Smith's observa- 

 tions seem to prove that P. formosiini and its congeners have 

 them longitudinal but wavy, which is a positive addition to our 

 knowledge, since we should naturally have expected them to be 

 oblique. The appearance of the finer square tessellation in either 

 of the principal films of an obliquely marked Pleurosigma would 

 seem to prove it to belong to the " quadrille " marked class, and 

 I think the smaller forms which Mr. Smith has left unnamed 

 may be identified as P. obsciiriim W. Smith, which is probably 

 only a small form of P. formosum or P. decorum. 



I do not find in the prints any conclusive evidence that the 

 quincuncial marked species, as P. angulatum, have the same series 

 of fibrils. No one doubts that all have a vegetable membrane in, 

 which the silex is deposited, and, under favorable circumstances, 

 a fracture through a row of dots would leave the thicker connect- 

 ing membrane looking approximately like a fibril. The argument 

 from analogy is not as strong here as in the case of the " qua- 

 drille " marked kinds. The structure may be found in all, but 

 the evidence does not yet seem complete. There is here a good 

 field for further investigation. 



This leads me to say that the size of the fibrils shown by Mr. 

 Smith does not seem to me so minute that any good yV o^" tV 

 objective should not define them. We must remember that the 

 condition of an object may count for much in the resolution of 

 its structure. A thickly silicified shell may not show what an 

 imperfectly silicified one will demonstrate. The former will 

 break into small angular bits with a mineral fracture; the latter 

 may separate into threads or membranes. The floating off of 

 the fibrils in print No. 15 seems to show that the shell was in a 

 peculiar condition; a sort of dissection of an uncommon kind 

 having taken place naturally or artificially. It would be an 

 interesting experiment to subject various species of Pleurosig/na 

 to the action of hydrofluoric acid for varying periods, and then 

 mount them for examination. To extend Prof. Bailey's old 

 experiments in this direction would be very useful; but the danger 

 of injury to the objective is such that it would hardly be advis- 

 able to watch the action of the acid under the microscope. 



