NOTES AND MEMOKANDA. 439 



middle into an hour-glass shape, and then divides in the centre into 

 two bodies of conical shaj)e, each with a nearly flat but toothed base 

 and a pointed apex. To a colourless elevation in the centre of the 

 nearly flat base are attached two long vibratile cilia ; the zoospores 

 thus ditfering considerably in form from any known elsewhere. 

 Various stages of this process are to be observed at one time within 

 the external membrane of the same mother-cell, which is now gradually 

 dclitiucscing, so that the zoospores are set free into the surrounding 

 water, where they move about for a time with a comparatively slow 

 motion, and then fall to the bottom. Their further development 

 Dr. Schmitz has been unable to follow. 



With regard to the systematic position oi Halosphcera, notwithstand- 

 ing its external resemblance to Volvox, its internal 'ructure forbids its 

 location among Volvocinese. It bears more resemblance to De Bary's 

 genus of ConjugatfB Eremospluera ; but until its life-history is more 

 completely known, its genetic affinities must remain in obscurity. 



Plate XVI. Fig. 1. — Single cell of Halospluera viridis ; the nucleus 

 has formed a great number of nuclei by repeated bipartition. 



Fig. 2. — The same. The parietal layer of protojilasm has col- 

 lected round the very numerous nuclei into flatly hemispherical 



Fig. 3. — The same. The outer cell-wall layer has burst after the 

 daughter-cells have been completely formed ; the inner layer 

 has stretched considerably ; within it the daughter-cells have 

 become separated from the cell-wall. 



Fig. 4. — Formation of the daughter-cells. The thin parietal layer 

 of protoplasm between the sei)arate hemispherical masses of 

 protoplasm is beginning to separate ; a number of cavities have 

 already made their appearance in it. 



Fig. 5. — Abnormal formation of two daughter-cells, whose nuclei 

 have nearly coalesced. 



Fig. 6. — Daughter-cells immediately after the division of the 

 mother-cell, still in close contact with the cell-wall. 



Figs. 7-9. — Division of the daughter-cells in the formation of 

 zoospores. 



Figs. 10, 11. — Zoospores. Fig. 10 in optical longitudinal section. 



I'ig. 12. — Division of a daughter-cell into four zoospores. 



Figs. 13-15. — Abnormal formations. Two (Fig. 15), five (Fig. 

 13), and twelve (Fig. 14) zoospores are formed by incomplete 

 division from a single daughter-cell ; they have become com- 

 pletely separated from one another. 



(Figs. 1-3 X 100 ; Figs. 4-11 x 300 ; Figs. 12-15 x cir. 150.) 



Black Mildew of Walls.— Professor Leidy, at a meeting of the 

 Philadelphia Academy of Natural Science, referred to an article in 

 ' Hardwicke's Science-Gossip ' for August by Professor Paley, entitled 

 " Is the Blackness on St. Paul's merely the effect of Smoke '? " Accord- 

 ing to the author, the blackness is mainly dae to the growth of a 

 hitherto undescribed lichen which appears to flourish on limestone, and 

 in situations uuaflectcd by the direct rays of the sun. Professor Leidy 



