162 Mr. G. J. Allman on a new genus of Algae. 



Connected with the Dock is a deep shaft partly filled with 

 water. The shaft is covered over with boards, so that the 

 light is almost totally excluded. Into this some of the green 

 substance had made its way, and on bringing up some of it 

 in a phial, I found that it differed from that in the open dock 

 in being of a duller green ; in other respects I could detect 

 no difference. I mention this as it illustrates an important 

 fact in the physiology of the green matter, and would appear 

 to establish the influence of light in the development of its 

 colour. 



Under the microscope this singular substance is seen to 

 consist of exceedingly minute, simple, moniliform threads, with 

 the globules composing them of uniform diameter, and the 

 threads themselves variously but elegantly curved, and grouped 

 together without order in a gelatinous matrix. 



The green substance of the Dock then is a minute Alga of 

 the family Nostochinae, and evidently comes very near to the 

 genus Anabaina of Bory Saint Vincent, and still nearer to an 

 Alga discovered by Mr. Thompson in Ballydrain lake in the 

 county Antrim, and which he names " Anabaina ? spiralis" 

 referring it doubtfully to Bory Saint Vincent's genus*. 



From Anabaina of Bory Saint Vincent, the present Alga, as 

 well as that of Mr. Thompson, differs in the uniform size of 

 the articulations, Bory's genus being characterized by larger 

 globules occurring at distinct intervals in the series. Were 

 the size of the filaments to be considered of generic import- 

 ance, there might perhaps be found in their great minuteness 

 in the plant now under consideration another distinctive cha- 

 racter. The mere size however of the filaments is scarcely of 

 sufficient importance to entitle it to the rank of a generic cha- 

 racter, yet, when taken in conjunction with others whose im- 

 portance must be admitted, it may materially assist us in 

 forming an opinion as to the real systematic rank of the Alga. 

 It must be recollected too, that in the minuteness of the fila- 

 ments the Anabaina impalpabilis is perhaps equally remark- 

 able, a fact, which, if the plant last named be a real Ana- 

 baina, would deprive this peculiarity of all value as a generic 

 distinction. 



The absence however of the enlarged articulations in the 

 Alga of the Canal Dock is a very obvious character, and upon 

 the whole I consider myself justified in establishing for it a 

 distinct genus, in which the Anabaina ? spiralis of Thompson 

 will also find a place. I suspect too that Bory's A. impalpa- 



* See a very interesting paper by Mr. Thompson " On a minute Alga 

 which colours the waters of Ballydrain lake near Belfast." — Ann. Nat. Hist, 

 vol. v. p. 75. 



