ARCHER AND DIXON, ON DESMIDIACE.E. 85 



Tetrachastrum. 



Frond longer than broad; segments constricted about two- 

 thirds of the way from the base ; lobes mucronate, their 

 extremities not bidentate. Mucronatum. 



Frond as broad as or slightly broader than long; segments 

 constricted about half-way from the base ; lobes not 

 mucronate, their extremities bidentate. 



Frond with convex ends ; lobes conical ; colour rich green. 



Oscitans, 



Frond with straight ends, plane ; lobes triangular ; colour 

 pale. Pinnatifidum. 



Notice of some Cases of Abnormal Growth in the 

 Desmidiaceje. 



It has occurred to me that the accompanying sketches, 

 representing an abnormal mode of growth in the Desmidiacese, 

 exhibiting, as they do, an appearance so curious and unusual, 

 might possess some interest for the students of that family. 

 I am aware that, in Mrs. Herbert Thomas's interesting 

 communication (' Quarterly Journal of Microscopical Science/ 

 vol. iii, plate v, figs. 17 and 18), that lady has figured a 

 very similar case in Cosmarium margaritiferum to that shown 

 in my drawing of Staurastrum dejectum ; yet I have thought 

 it might be worth while to figure some examples of the 

 phenomenon still farther carried out in other genera, al- 

 though it may be quite possible that even more curious 

 aberrations may have been met with by other observers. 



The first case of this mode of malformation to which I 

 shall direct attention is a monstrosity of a variety of 

 Micrasterias Jenneri (Plate VII, fig. 9). Here the inter- 

 vening growth, produced after the mode which prevails in 

 the Desmidiacese, between the two older segments of the 

 original frond, and which, in the normal condition, ought to 

 have formed two new segments, forms a somewhat quadrate 

 expansion, but has not assumed any definite outline. We 

 find it within filled with endochrome, similar to the parent 

 segments, to about the dimensions of one of which it has 

 attained. It is about the simplest form of the irregularity 

 under consideration which I have to bring forward ; the 

 intervening new portion forming only an irregular, shapeless 

 growth. 



I here wish to draw attention in passing to the form itself 



VOL. VIII. II 



