144 '^^^ Irish Naturalist. June, 



C. fragilis and its var. delicatula, C hispida, C aspcra subsp. 

 desmacantha, C. tonie^itosa, of course, aud, very sparsely, 

 C. polyacantJia. 



One point of interest seems worth mentioning. Several of 

 the streams in the locality of Carrickmacross, where the 

 Characeae yield is richest, showed along their banks, under a 

 thick bed of black peat, a substratum of white shell-marl of 

 considerable thickness. The same occurrence is frequent in 

 our fen-lands of Cambridgeshire, and in the Memoirs of the 

 Geological Survey for Cambridgeshire and Suffolk (Ely, 

 Mildenhall and Thetford Section), there is an interesting dis- 

 cussion on the characteristics of this shell-marl and its origin. 

 " The greatest thickness," writes Mr. Skertchle}^ " is three 

 and a quarter feet, but for more than two square miles it is 

 not less than two feet. The pure white of the marl, and the 

 dark colour of the overlying and underlj-ing deposits, makes 

 the sections very striking. Not a stone or patch of cla}^ or 

 other foreign matter, can be found, and the fresh-water shells 

 seem to be pretty constant throughout. The mass is jointed 

 so as to form large lozenge-shaped masses, and the main 

 joints are very regular. The roots of Chara can be seen in 

 it, and the seeds obtained by careful examination. Indeed, I 

 have no hesitation in attributing the formation of the shell- 

 marl to the decay of Chara of various species. These plants 

 are still living in the neighbouring dykes, where, so far as my 

 observation goes, they form dense masses, to the exclusion of 

 other plants. The axes of two species. C vulgaris and 

 C hispida are encrusted with carbonate of lime. . . . This 

 forms a deposit at the bottom of the water. One season's 

 deposit is usually only a white stain, but under exceptional 

 circumstances an appreciable thickness is formed." If Mr. 

 Skertchley's theory be correct, the deposits of shell-marl 

 round Carrickmacross indicate extensive tracts of water in 

 the neighbourhood abounding in Characeae. Probably many 

 of the lakes covered a much larger area than at present, and 

 the deposit of shell-marl marks their original extent. 



In brief, then, my visit resulted in proving that Monaghan 

 is not behind other Irish counties in the number of Characecs 

 which it yields, and that it possesses one species not so far 

 recorded from Ireland — Nitclla viucrojiata Kuetz., while the 



