1G2 



On the Stkucture of the Nodules in PleurosigmyE, 



{Glwiacosphenia monilvjera.) 



By Edward M. Nelson, P.R.M.S. 



{Read December 16th, 1898.) 



Plate 10. 



While examining the Fleitrosiymw in the Nottingham (Mary- 

 land, U.S.A.) deposit I was much struck with a marked difference 

 in the structure of the nodules when compared with those of 

 recent forms. Now, as the Nottingham deposit belongs to the 

 Middle Tertiary Period, it cannot fail to be interesting to compare 

 these with other fossil and recent forms. 



Fig. 1 shows the raphae pipes entering the nodvile of a Notting- 

 ham Pleurosigma, as seen from the outside of the valve. You 

 will notice that both ends of the pipe, after making a slight dip, 

 terminate with their extremities pointing directly at one another. 

 Fig. 2 is a very highly magnified picture of the extremities of the 

 pipes ; they are joined by a very fine thread-like structure, which 

 by the way is an excessively difficult image. Passing on now to 

 the nodule itself, we come in fig. 3 to an inside view at the highest 

 focus ; fig. 4 is the same at an intermediate focus ; focussing down 

 still lower we come to an inside view of fig. 1, which being similar 

 to it renders another figure unnecessary. 



Turning now to a P. angulatiLin, we have in the outside view 

 at fig. 5 the extremities of the pipes very differently arranged. 

 Here we see the left-hand pipe slightly bent down, while the right- 

 hand one is considerably turned to one side. Passing on to an 

 inside view, fig. 6, we also find a totally different structure : the 

 nodule has become oval, with the raphae pipes running a little 

 way into it at each extremity. Focussing down, we come to the 

 inside view of fig. 5, there being no intermediate image, as was 

 the case in the fossil form. 



In fig. 10 the ends of the raphse pipes of a Nottingham 

 pleurosigma are shown greatly magnified, and it will be seen that 



