Calcareous Pebbles formed by Algcs. 77 



centre, and they would form a more compact rock than the Michigan 

 pebbles. Mr. Dyer, in a short examination of the algal contents, determined 

 them to be a Rivularia with Scytonema, and Mr. Archer had also noted 

 other Algae casually interwoven in the mass. I have examined it with 

 much care and have compared it with the Michigan specimens, and the 

 predominant Alga is the same in both cases viz., Schizothrix fasciculata. 

 As in the Michigan pebbles, there are a number of other Algae casually 

 present notably a species of Dichothrix ( = Scytoneina) as observed by Mr. 

 Thiselton-Dyer, Nostoc cells, diatoms, &c. I have found no trace of the 

 CladopJiora, nor of anything resembling one, mentioned by Mr. Barber in 

 Annals of Botany (vol. iii., p. 144), and Mr. Thiselton-Dyer (loc. cit^ also 

 has seen nothing to confirm the statement. 



It is much to be wished that some of our Irish botanists would give 

 an account of the growth of these bodies. They are in shallow water in 

 both cases, and may be a product of the high temperature of summer ; 

 but speculation on this point is useless without further knowledge. Mr. 

 E. Grove has kindly examined the Lough Belvedere pebbles for diatoms, 

 and his lists, 'allowing for the differences between the diatomaceous flora 

 of North America and Great Britain, show much resemblance between 

 the two gatherings.' 



GEORGE MURRAY. 



EXPLANATION OF PLATE XIX. 



Fig. i. Pebbles from Michigan natural size. 



Fig. 2. 



Fig. 3. in section 



Fig. 4. Interwoven filaments of Schizothrix fasciculata ( x 86), from Michigan 



pebbles. 



Fig. 5. a, b. Filaments of pebbles (a x 232 ; a 1 x 375 ; b x 232 ; & x 375). 



Fig. 6. Pebbles from Lough Belvedere, Mullingar. 



