ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 783 



presented also anotlier departure from the normal structure, the 

 cystocarps being in the form of normal favella3, instead of, as is 

 usually the case, in that of lateral seirospores. The species was, 

 however, well distinguished by the ampuUiform terminal articulations 

 of the branches. 



Congenital union of growth on the Thallus of the PoUexfeniese.* 

 — The apical growth of the Ehodomelese has been shown to depend 

 on the segmentation of a single apical cell. Falkenberg has, never- 

 theless, observed in the Pollexfenieje — a group consisting of the 

 genera Pollexfenia, Jeannerettia, and PlacopJiora — that the thallus 

 grows, along its entire unilamellar anterior margin, by means of an 

 apical angle ; the different marginal cells showing also different 

 modes of division. A close examination shows that we have not 

 here an apical angle composed of cells originally of equal value, as in 

 Taonia, Padina, or Peyssonnelia ; but that the growing margin of the 

 flat thallus is composed of the apical cells of unequal branches ot a 

 much-branched Polysiphonia-like system, all the branches of which 

 that lie in the same plane having congenitally united in their growth 

 through their whole length. 



Structure of Terpsinoe.t — 0. Miiller has already shown that in 

 the diatom-genus Epitliemia there is a system of septa which divides 

 the space enclosed in the shell into compartments. In addition 

 there is another septum, called by him the intermediate plate, which 

 divides the cell-cavity in two along its longest diameter. The same 

 he now states to be the case with Terpsinoe musica and americana. 

 In these species the intermediate plate remains in a very rudimentary, 

 scarcely perceptible condition. The margin of the shell projects in 

 a cushion-like manner into the cavity to meet the septa. A full 

 description is given of the number and relative position and size of 

 the septa. 



With regard to the question whether this genus conforms to the 

 Macdonald-Pfitzer hypothesis that the shell is double, the author 

 points out that the optical image of the boundaries of the hoop is 

 very defective, and that a refutation of this hypothesis can best 

 be obtained by simply observing the microscopic image of the margin 

 of the hoop. 



The superposition of the hoop may exist without being 

 discernible in the optical section of the lateral margin. The 

 section of two membranes of the same substance, and therefore the 

 same refractive power, which lie so closely on one another that 

 another refractive medium cannot be interposed, necessarily gives 

 the impression of a single membrane of double thickness. But 

 when a differently refracting medium appears at the free edge of 

 the superposed membrane, and extends a little way, the appearance 

 is produced as though a lamella had been split off from the otherwise 

 homogeneous membrane. This is precisely the appearance of the 

 margins of Terpsinoe ; such apparently split-off lamellae frequently 



* Gottinger Nachr., Dec. 15, 1880. See Bot. Ztg., xxxix. (1881) p. 159. 

 t SB. Gas, naturf. Freunde Berlin, 18th Jan., 1881, 14 pp. 



3 F '2 



