ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 499 



more or less delicate punctations on the inner face. It has been 

 impossible to ascertain whether these punctations completely traverse 

 the lower layer of the valve, or only go to a certain depth. In Tri- 

 ceratium Favus, of which Dr. Van Heurck succeeded in artificially 

 obtaining transverse fractures of the greatest delicacy, he was not 

 able to see any perforation, although the denticulated form of a mem- 

 brane found broken in the interior of an alveole seems to be in favour 

 of a real perforation. In certain Coscinodhci the valve seems 

 thoroughly perforated below, and this perforation is undoubtedly 

 evident in the fossil forms from Jutland. In Eupodiscus Argus there 

 are tolerably large perforations (?) in the lower layer (these are the 

 large granules which are seen in the alveoli) ; and besides these there 

 are very fine granulations. 



The upper layer of the valve exists in all degrees of development. 

 In its most perfect condition, in Triceratium Favus, Coscinodiscus 

 Oculus-Iridis, &c., it forms the alveoli usually closed from above. In 

 the next condition of lesser development {Eupodiscus Argus) the 

 alveoli remain open above. In carrying on researches through the 

 series of genera and species, the upper layer is seen diminishing in 

 strength, and the weakest points, the walls of the alveoli, first dis- 

 appear, then the intermediary portions between several alveoli 

 diminish, and end by being reduced to the condition of small spines 

 such as are found in so many species of BiddulpMa. Ultimately 

 these last vestiges of the layer of alveoli disappear in their turn, 

 and there only remains the lower punctated layer of the valve. 

 Triceratium 0)itylum) intricatum, for instance, shows this layer 

 reduced to its simplest expression. 



The valves of the Pseudo-raphidese and of the true Eaphidese 

 show the same structure as the Crypto-raphide®, but the alveoli are 

 very much smaller. By Smith's medium their existence can be 

 shown in numerous species, and particularly in the genera Baphoneis, 

 NitzscMa, and Pleurosigma. 



The beads of diatoms are therefore, in reality, formed by alveoli 

 and striae, and are often formed by the walls of these alveoli. The 

 valve is said to be alveolated or to be of cellular structure when 

 these alveoli are large and appear to be polygonal ; they are said to be 

 beaded or punctated when the alveoli are more or less diminutive. 



Lichenes. 



Development of the Apothecia of Lichens.* — Dr. M. Funfstiick has 

 followed the development of the apothecia in three genera, Peltigera, 

 Peltidea, and Nephroma. In Peltigera canina an interval of several 

 years passes between the first formation of an apothecium and the 

 production of the first ripe spores. In Peltigera and Peltidea there 

 are no spermogonia. 



In Peltigera malacea the apothecia originate as extremely 

 minute roundish dots on the margin of the thallus, where a few fila- 

 ments, irregularly coiled into rosettes, the ascogonia, are formed close 

 beneath the cortical layer, on a level with the gonidial zone ; they 



* Jahrb. K. Bot. Gart. Berlin, iii, (1884) 20 pp. (3 pis.). See Bot. Centralbl., 

 xxi. (1885) p. 69. 



2 K 2 



