MEMORANDA. 309 



same diatom. Other instances might be mentioned in whiclj 

 the total number of striae is more constant than the iiumber in 

 '001 in. It is obvious that if the number in "001 in. be constant 

 at all ages, we must allow that in passing from the youngest 

 state to maturity the valves have the power of adding new 

 striae as they grow larger, but whether tliese additions are at 

 the extremities or at the centre no one has taken the trouble 

 to investigate. It niay be alleged that the new valves formed 

 by the process of self-division acquire more numerous striae 

 than before ; but while it is generally conceded that one of 

 the old valves remain in each frustule, this explanation can 

 scarcely be admitted, unless we are prepared also to allow 

 that there may be a great dissimilarity in the striae between 

 the two valves of the same frustules. If the total number of 

 stria^ in the valves remain constant, we must suppose the striae 

 to separate slightly as this valve enlarges ; and this supposi- 

 tion appears to me more in accordance with observations 

 hitherto recorded, although, no doubt, such observations have 

 not been made directly in reference to this point. 



If the number of striae on the entire valve be found more 

 constant tlian the number in a given space (and here some 

 allowance must be made for the produce of different sporangia 

 not being identically the same either in size at the same age, 

 or in the precise number of striae to each valve), we may often 

 have a criterion whereby to decide whether a frustule be the 

 young state of a species or a distinct species or variety. I 

 have alluded to the ^ and y of P. Balticum not being 

 entitled to rank as varieties, any more than a lamb is a distinct 

 variety of a sheep ; but, on the other hand, il we examine 

 Nitzscliia duhia of Smith, we shall find not only that his /8 

 is much smaller than the a, but that while the striae of a are 

 difficult to be resolved by a i those in /3 are readily made out, 

 being more distant. The small state here has more distant 

 striae than the large one, and although sometimes mixed in the 

 same gathering cannot possibly be the young of the other ; it 

 may form a peculiar variety, but my observations tend to 

 allow it the rank of a species, if indeed it do not belong to a 

 different genus. 



Sufficient attention has not yet been paid to the sporangia!, 

 state of the diatoms ; from the observations recorded by 

 Thwaites, Smith, and others, different genera seem to follow 

 diff'erent laws on the subject. In Navicula this state appears 

 to be always accompanied by a great dilation of the frustule, 

 and the formation of a strong line or band between the median 

 line and the margin ; sometimes the new line is nearly straight 

 and parallel to the median line except near the nodule, with 



