TJie Structure of Eupodiscus Argus. Ill 



granulation. It is covered with round dots, radiating irregularly 

 from the centre, and leaving irregular blank spaces between the rows. 

 It is probably this surface that is figured by Mr. Slack, who makes 

 no mention of any difference between the two surfaces, but appears 

 to have made the drawing from a specimen on M oiler's typen 

 platte. In my typen platte there are the eighteen -corner Eupo- 

 disci, and three others, and all were mounted concave side up, which 

 is the easiest mode of making them stay in place. 



A difficult question is now met, and that is, what is the nature 

 of these dots ? The solution of this question affects Mr. ' Slack's 

 theory of the depositions of silica in spherules, and also the inter- 

 pretation of appearances given by Mr. Wenham's " Keflex Illumi- 

 nator." If we concede these dots to be spherules, then Mr. Slack 

 has gained one side of the valve only, and must further show that 

 the upper side is covered with them ; perhaps he can succeed in 

 this ; but I think he must fail on the processes, which are as clear 

 and smooth as glass rods. 



If we examine these dots by light both from above and below, 

 the transmitted light passes through the large depressions on the 

 convex surface, and thus through four or more of the dots on the 

 inner surface, making them brighter than their neighbours. We 

 have then the appearance figured by Mr. Slack. 



I can see them with a -jV, but get a very distinct view with 

 a -ziy, and Professor Smith's apparatus, and to my eye, with that 

 light, they are slight depressions, like dents, with a white spot in 

 the centre as if the bottom of the dent were slightly convex. The 

 average diameter is about ^^oV- 



I have tried to represent them at B in a fragment more highly 

 magnified than that at A. 



The examination of other species of this genus cannot deter- 

 mine the structure of the E. Argus, for we cannot affirm the 

 identity of structure of two species until we have satisfactorily 

 determined that of each. 



I have no opportunity of using the " Keflex Illuminator," and 

 therefore send with this a slide of fragments mounted dry, which I 

 think will prove instructive to anyone who wishes to investigate 

 the subject, and has access to different modes of illuminators. 



Boston, Mass., U.S.A. 



[The reader should refer to Mr. Slack's letter on this subject 

 in the present number.— Ed. ' M. M. J.'] 



VOL. IX. 



