ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 411 



the corners must compel us to regard this form as a Xanthidtum, of 

 which genus it would certainly be the most minute species, and might 

 stand as XantJiidium concinnum. 



Structure of Diatoms in the Jutland "Cement-stone."* — Dr. 

 E. van Ermengem gives the results of the investigations of himself 

 and M. Prinz on this subject. The phenomena of diffi-action pro- 

 duced by the structural elements of the valves of some very finely 

 marked diatoms, such as Pleurosigma, prevent an exact appreciation 

 of their form. The hemispherical granulations which, in the opinion 

 of most diatomists, would produce these markings, do not esist ac- 

 cording to others, and Prof. Abbe himself thinks that they are not 

 due to elevations or spherules. Neither is it probable that all diatoms 

 have the same structure ; 0. Miiller, A. Schmidt, and Flogel admit as 

 many as four or five diiferent tyj^es. 



In their researches the authors have used those species whose 

 structure is least delicate, such as Coscinodiscus Oculus-Iridis Ehrg. 

 and Trinacria Begina Heib., which are very abundant in the diatomi- 

 ferous rock at Fiir (Jutland). In studying the valves in media of 

 various indices of refraction, they obtained a series of optical reactions 

 which indicate that the valves are perforated with very fine apertures. 

 The results of this examination entirely agree with those of other 

 methods of research, especially with those given by the study of 

 sections of the frustules made in different directions. The calcareous 

 rock in which they are imbedded is well adapted for polishing and 

 the preparation of exceedingly thin slices ; and the normal or oblique 

 sections of the diatoms obtained by these means can be studied in the 

 calcareous matrix or in different media after the solution of the calcite. 

 It is easy to avoid all rough manipulation which might alter their 

 structure. Mounted in a liquid of high refractive index (l- 68), such 

 as the saturated solution of iodide of mercury in iodide of potassium 

 (Stephenson), they give images of remarkable clearness. It is evident 

 on an examination of the slightly oblique sections, that the valves of 

 Coscinodiscus are composed of two layers ; the upper layer shows 

 hexagonal alveoli, the lower layer is formed by a very thin mem- 

 brane perforated with very small circular apertures. The apertures 

 are surrounded by a thickened annular edge. This layer, when de- 

 tached from the subjacent calcite by the wearing away of the rock, 

 leaves very evident impressions, which do not correspond either with 

 the convexities or the concavities of its lower surface. 



The thinnest normal sections, the thickness of which is less than 

 the half of an alveolus, prove still more clearly the existence of aj)er- 

 tures in the lower layer : the membrane which closes the bottom of 

 the alveoli is manifestly interrupted in its centre, and this lacuna is 

 included on every side between three swollen portions whose section 

 more or less resembles a crescent. 



Sections of Trinacria show also the existence of pores traversing 

 the entire thickness of the siliceous envelope. 



The appearance presented by the double connectives of Goscino- 



* Bull. Soc. Belg. Micr., ix. (1883) pp. 53-7. 



