Ml*. J. Ralfs on the Nostocbinese. 325 



the employment of old names, or, renouncing the task as hopeless, 

 to invent new ones. In the present group all these difjiculties 

 have been experienced, and unless authentic specimens, in a fit 

 state for examination, exist in the collections of liinnieus and 

 other early botanists, it must in some instances be impossible to 

 affix their names with any certainty. I cannot flatter myself that 

 my nomenclature will be free from error ; but I venture to hope 

 that by })ointing out the essential peculiarities of these plants, 

 and by more detailed descriptions of the species, I shall facilitate 

 the labour of those who can find opportunity for inspecting the 

 herbaria of original authorities. 



Should I succeed in my endeavours to elucidate the British 

 species, the success will be due in a great measure to the kind 

 assistance of my fellow- students. Mr. Thwaites, who in Harvey's 

 * Phycologia Britannica ' was the first to recognize three kinds of 

 cells, has supplied descriptions of some species which I ha\^ not 

 met with. Other friends, e&pecially Professor Allnian, Mr. An- 

 drews, jNIr. Jenner, IMr. Moore, the Rev. T. Salwey, and Mr. W. 

 Thompson, have aided me by specimens accompanied with re- 

 marks ; whilst Mr. Borrer has enabled me, by means of his rich 

 botanical library, to clear up points on which I must otherwise 

 have remained in doubt*. 



* The following synoptical table will, it is hoped, convey a clear idea of 

 the characters distinguishing respectively the various genera j)roposcd to be 

 described. The genus Nostoc is not included in the i)resent i)aper on ac- 

 count of the necessity which exists of a further examination and study of 

 its several species, some of which have been stated by Professor Kiitzing 

 and M. Fries to be merely a condition of species of Collema. 



Synoptical Table of Genera. 



I. Filaments not included in a membranous sheath. 



a. Frond definite. 



1 . Monormia, Berkeley. 



b. Frond indefinite. 



2. Trichormus, Allman. Vesicular cells interstitial and terminal. Spo- 



rangia formed first from the cells at the greatest distance from the 

 vesicular cells. 



3. Sphcsrozyga, Ag. Vesicular cells interstitial. Sporangia formed 



first from the cells nearest the vesiculai* cells. 



4. Cylindrospermum , Kiitzing. Vesicular cells terminal. Sporangia 



as in Sphcerozyga. 



5. Dolichospermum, n. gen. Vesicular cells interstitial. Sporangia 



without any definite arrangement, and of unequal length. 



II. Filaments included in a membranous sheath. 



6. Aphanizomenon, Morren. Vesicular cells none ? Sporangia usually 



single and of unequal length . 



7. Sper^mosira, Kiitz. Vesicular cells interstitial, single or sometimes 



two together. Sporangia as in Trichormus. 



