S08" MEMORANDA. 



only to the mature iVustule, seems really to exist at a very 

 early age, as soon indeed as a frustule is formed from the 

 sporangium. Between the young state and the fully-formed 

 or sporangial one there must thus be a considerable difference 

 in size ; and, as the number of frustules (the produce of one 

 sporangium by self-division) increase in a geometrical pro- 

 gression, we may always expect to find many more of the larger 

 than we did a short time previously of the smaller size, in the 

 same pool. All the frustules from the same sporangium 

 exhibit probably nearly the same size at the same age ; but in 

 the same locality other sporangia may have been deposited, so 

 that we find the same species of different ages or sizes mixed 

 together. The minimum and the maximum state is therefore 

 one of the points that require to be decided on by naturalists 

 before we can form a definite idea of any one species. 



The strias on the valves of a species requires also to be 

 studied ; and here the question arises, is the distance between 

 the striae, or their number in "001 inch, constant ? or does 

 the distance vary with the size of the valve, the number on 

 the entire valve being constant, when the frustules are all the 

 produce of the same sporangium ? So long as this remains un- 

 determined by actual observation, it is of no use counting the 

 number of striae in a given space, or of talking of any species 

 being a test for object-lenses. Mr. Sollitt, of Hull, in giving 

 the number of striae of Pleurosigma quadratum and angulatum 

 (Microscopical Journal, II., p. 62, when by P. angulatum he 

 meant P. quadratum, and by P. strigosum is meant P. angu- 

 latum of Smith), points out a considerable difference between 

 the distance of the striae in small and large specimens, and 

 that they are more distant in the latter ; as however he does 

 not give the exact length of the frustules, nor say that the 

 valves were found in the same gathering, or if obtained from 

 the same locality at some weeks' distance of time, no positive 

 conclusions can be drawn from his measurements. So far as 

 my own observations go, the number of striae in the entire 

 valve is tolerably constant, whether small or large, when ob- 

 tained at the same season and from the same locality ; and 

 consequently the actual number in '001 in. is of less conse- 

 quence than generally supposed, unless we multiply that 

 number by the length of the valve. Smith's fi and y of Pleu- 

 rosigma Balticum seem to have nearly the same total number 

 of striae, as in his a ; but the first is about half the size of y, 

 and the other about one-third ; so the striae in these ought to 

 be from two to three times closer than in a, and this agrees 

 well with observation : these two, then, cannot be considered 

 in the light of varieties of a, but merely a younger state of the 



