452 Dr. Henri Blanc on Ceratium hirundinella. 



animal being seen from the dorsal surface, is no longer a liorn, 

 but only a simple stout spine (fig. 4). 



Usually, when this third horn is well developed and quite 

 evident, the other two posterior horns, which are always 

 longer than thisj diverge more from one another ; while if it 

 is only rudimentary, the two posterior horns are less diver- 

 gent, and sometimes even nearly parallel. (Compare figs. 4, 

 6, and 7 with fig. 5.) Dr. Imhof * has found in the Katz- 

 ensee C&j^atia closely resembling his G. reticulatum^ and only 

 differing from it by a small eminence situated outside of the 

 left posterior horn. This is what he says about it : — " Per- 

 haps this species from the Katzensee might be regarded as an 

 intermediate form between Ceratium liirundinella^ Miiller, and 

 our first form." 



It is to be regretted that M. Imhof had not abundant 

 material at his disposal, for he would certainly have been able 

 to convince himself that the Ceratia of the Katzensee were 

 really an intermediate form. He would have seen that 

 between the actual well-developed horn and the simple promi- 

 nence there exist all the passages. 



As regards dimensions the Ceratium of the Lake of Geneva 

 does not differ from those of the lakes of Zurich and Zus:, nor 

 from those found by M. Bergh in the fresh water of the 

 neighbourhood of Copenhagen. Measured upon several 

 individuals the distance between the extremity of the anterior 

 horn and the extremity of the left posterior horn (the indi- 

 vidual being looked at from the dorsal surface) varied from 

 0"267 to 0*190 millira. All the specimens observed, whether 

 with three or two posterior horns, had the same ventral aper- 

 ture, 0'014-0*015 millira. in width, that is to say, of the same 

 dimensions that M. Imhof gives for his specimens. I may 

 remark further, with regard to the buccal aperture, that in 

 fact the figure given of it by M. Bergh for Ceratium hirundi- 

 nella is not exactly what M. Imhof and myself have observed; 

 but I cannot attach any great importance to a simple diver- 

 gence, which may originate merely from a defect in the 

 drawing. All the Ceratia of the lake had also the same 

 cuirass, the description of which given above agrees exactly 

 with those given by MM. Bergh and Imhof of the cuirasses 

 of the Ceratia which they had before them. All had the 

 same protoplasm containing the same nucleus. Nuclei in 

 course of division occurred in specimens with two posterior 

 horns and in others with three horns. 



This uniformity and constancy in the principal characters 



* Imhof, he. cit. p. 167. 



