GG4 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
which are liable to our criticism, there still remain several which can- 
not be thus disposed of. 
Prints Nos. 14 and 15, taken with half the magnification of most of 
the others (x 875), show strips of surface marking which strongly sup- 
port Mr. Smith’s interpretation, viz. that the outer surface of P. for- 
mosum is covered by a longitudinal series of fibrils separating so as to 
pass round the alveoli and uniting over the solid corner interspaces. 
The definition in these cases is not only reasonably clear and free from 
the ordinary marks of diffraction effects, but, most conclusive of all, 
there is in No. 15 a bit of this film floated off the shell and lying 
detached by its side. The fibrillar structure of this bit leaves little 
room for scepticism, and it so exactly accords with the appearance of 
the similar fibrils remaining on the surface of the shell that I cannot 
refuse to accept it as evidence of structure. Going back from these to 
prints Nos. 10 and 11, we now find reason to accept these also as evi- 
dences of the same structure, though distorted by obliquity of light, so 
that they would not have been satisfactory taken by themselves. On 
No. 5 also we may recognize some of the same fibrils. The single de- 
tached fibril in No. 9 is not so directly connected with any other 
specimen, either in the photograph or in Mr. Smith’s description, as to 
present the evidence on which it is shown to be part of the same struc- 
ture ; but the measurement of its flexures so corresponds with the 
areolee of the shell that its probable connection with a similar valve 
may be assumed. 
The interpretation of this structure which seems to me most satis- 
factory is to regard these fibrils as superposed upon the general surface 
of the shell as a protection to the thin capping of the alveoli against 
abrasion. It would, in that case, come under the description of those 
appearances which I have referred to in paragraph 4 of my general 
summary, viz. a “ thickening on the exterior of the lines bounding the 
areolae .... which is not in contravention of, but is in addition to,” 
the usual formation of the shell by means of two principal plates or 
films. All the species of Pleurosigma which have the alveoli arranged 
in Brebisson’s quadrille seem to have strengthened ribs between the 
rows of “ dots ” — P. balticnm , P. attenuatum , &c., have them longitudinal 
and straight. Mr. Smith’s observations seem to prove that P. formosum 
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. obscurum 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 quin- 
cuncial 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 favourable circumstances, a fracture through a row 
of dots would loave the thicker connecting membrane looking approxi- 
mately like a fibril. The argument from analogy is not as strong here 
as in the case of the “quadrille ” marked kinds. The structure may be 
