Structure of the Diatom Shell, &c. By Br. J. D. Cox. 401 



examination of these species as opaque objects and with tlie vertical 

 illuminator is therefore restored to its full force and weight, 

 undiminished by the fact that such films are not always, or even 

 usually, seen in observations by means of transmitted light. 



The hoop of Isthmia furnishes us with evidence, also, upon 

 another point which has very naturally been the subject of dispute : 

 I mean the permeability of the films covering the alveoli, by the 

 media used in examming and mounting the specimens. It was long 

 since noticed that before the laminae of the Isthmia hoop slide upon 

 each other in the growth of the diatom, the areolae of the dots are 

 concentric, and but one dot will be seen where, as soon as the 

 sliding begins, two are plainly visible. In examples of the more 

 strongly marked sort, I have been able to detect the mottled film 

 which covers the alveolus, in both the outer and the inner laminae. 

 Before the sliding process begins, therefore, we certainly have at 

 least one, if not two, completely inclosed areola between the 

 siliceous films. This will be conceded by those who hold that the 

 areolae in each separate lamina are not completely inclosed, as well 

 as by those who believe that they are ; for two pitted surfaces, 

 however applied to each other, would make inclosures of these pits. 

 Yet, when the Isthmia is mounted, the dots of the thickest hooj), 

 where no sliding has occurred, are as free from air-bubbles or 

 evidence of vacuum as any other parts of the diatom. It would 

 seem to follow necessarily, therefore, that the medium has penetrated 

 these little cavities, and if in this instance it does so, the supposed 

 difiiculty on this score vanishes also in the case of Coscinocliscus, of 

 Triceratium, and the rest. A majority of observers appear to have 

 been convinced that this permeation of the frustule walls must 

 take place, since whole and unbroken frustules are found to be 

 filled with the mounting medium ; but the proof above indicated 

 may be of service in removing doubts in the, minds of some who 

 have still remained unconvinced. 



"Whilst thus presenting examples from Isthmia nervosa as 

 demonstrative evidence of the character of the films covering the 

 alveoli in diatoms, I wish to be understood to look upon it as 

 cumulative proof only, and by no means as admitting that the 

 direct examination of valves of Coscinocliscus oculus-iridis and 

 Triceratium favus is not enough to establish with reasonable 

 certainty the existence of the outer and inner films in those species. 

 The concentric circles of pink colour seen in the " eye-spots " of the 

 largest specimens of Coscinocliscus are a distinguishing mark sepa- 

 rating these from the homogeneous appearance and even tint of the 

 Polycystinae openings, and are as real if not as manifest evidence of 

 a film as the elaborate sculpture in the similar film of Isthmia. As 

 to that which covers the outer (convex) surface of Coscinocliscus, it 

 has now been observed and figured in so many species that although 



Ser. 2.— Vol. V. " 2. D 



