Prof. Gregory on British Freshwater Diatomacese. 73 



May 10. — Professor Balfour, President, in the Chair. 



Mr. P. S. Robertson exhibited germinating plants of the following 

 species of Coniferse, to show the remarkable variation in the number 

 of their cotyledonary leaves : — 



" Pinus nobilis ; normal number of cotyledons 6, varying with 4, 5, 

 and 7. 



'* Pi?ius Sabiniana-y 14, 15, 16 prevailmg numbers; variations 

 13, 17, 18, 19. 



" Pinus Jefrej/i ; prevailing numbers 9 and 10, varying with 7, 



8, 11. 



^^ Abies Hookeriana'f usual number 4, varying from 3 to 5. 



" Pinus Beardsleyi ; prevailing numbers 6 and 7, varying with 3, 

 5, 8, 9, and 10. This species occasionally produces two perfect 

 plants from one seed. 



" Thuja Craigana {Libocedrus decurrens) ; usual number 4, 

 varying from 1 to 4. 



" Cryptomeria japonica ; usual number 3, varying from 2 to 4. 



^^ Pinus Lambertiana ; usual number 14, varying with 10, 12, 

 and 13. 



" Pinus monticola ; usual numbers 8 and 1 0, varying with 6, 7, 



9, 11." 



The following papers were read : — 



1 . " On some new species of British Freshwater Diatomacese, 

 with remarks on the value of certain specific characters," by Prof. 

 Gregory. 



After some remarks on the distribution of Freshwater Diatoms, 

 the author proceeded to consider the value of certain specific 

 characters. 



Species, among Diatoms, are generally distinguished by the follow- 

 ing particulars, viz. the form ; the structure, where anything re- 

 markable occurs ; the length of the individual frustule, within the 

 usual limits ; the arrangement and number of the striae, where these 

 are visible, as well as their nature, whether moniliform or continuous, 

 narrow or broad, close or distant, &c. ; and frequently the aspect of 

 the median line, if present, and of the nodules at its centre and 

 extremities. 



The form or outline. — This varies so much, that, if we were guided 

 by it, we should make many false species, as is shown by the fact, 

 that these forms pass by gentle gradations into each other. This 

 kind of variation occurs, for example, in Navicula lacustris, of which 

 two very different forms occur ; but there is a third which is precisely 

 intermediate. It is seen also in Navicula ellipticay some forms of 

 which are oval, but of different proportions, others are constricted. 

 Navicula dubia is believed to belong to the same species as N. am- 

 phigomphus and N. dilatata, and by some persons, all the three are 

 united to iV. jirma. It is certain that all four agree in having 

 the side lines, but they all differ in outline. Navicula lepida, a new 

 species, exhibits three varieties, differing in form. But the most 

 remarkable ej^ample is found in Navicula varians. 



