1891.] NEW-YORK MICROSCOPICAL SOCIETY. 85 



effect, the fluid passing along the structural lines, obliterating 

 part and leaving part. 



But when full weight has been given to all these things, and 

 we have put aside those of Mr. Smith's long and beautiful series 

 of photographs which are liable to our criticism, there still remain 

 several which cannot 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 support Mr. Smith's interpretation, viz., that the outer 

 surface of P. formosiim 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 dif- 

 fraction 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 

 print Nos. 10 and 11, we now find reason to accept these also as 

 evidences 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 detached 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 structure; but the measure- 

 ment of its flexures so corresponds with the areolae of the shell 

 that its probable connection with a similar valve may be assumed. 



The interpretation of this structure which seems to me most 

 satisfactory 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 (see foot-note, p. 75 ante), 

 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 Pleurosigiiia which have the 



