208 Proceedings of Royal Society of Edinburgh. [sess. 
lengthening of the frustule, hut secretes a siliceous envelope and 
becomes a gonidium or sporangium, consisting of a cell with two 
rounded ends, and a connecting hoop, one end being smaller than 
the other I may mention here that these sporangia (if one 
may call them so) of Bacteriastrum and Chcetoceros are very 
variable in form and size, and, from having been found in a free state 
in deposit or guano, have been very often mistaken for distinct 
species of diatoms by many observers.” Again he says (p. 76), 
after describing the ordinary division in Chcetoceros , “The con- 
densed endochrome, instead of becoming paler and dividing, 
gradually assumes another shape, varying with the species, and 
secretes a siliceous envelope The contents of the sporangium 
soon escape, but I have not been able to follow out the further 
processes they undergo towards the reproduction of a Chcetoceros .” 
Castracane, in describing the “Challenger” diatoms, found a 
form known as Dicladia capreolus within the frustules of Chce- 
toceros, and Mr Thomas Comber * has quite recently described 
the endocvsts of Thalassiosira antarctica. It appears to be 
established that in these genera there occurs a kind of dimorphism, 
possibly an alternation of forms hitherto believed to belong to 
different genera. 
Finally, Professor Cleve f has figured, in his Diatoms of the 
Arctic Sea, one Biddulyhia aurita within another. He observes 
respecting it : “In one specimen from Greenland I have observed 
a sporangial frustule. The large exterior cell measured 0*066 mm. 
in height, and 0*044 mm. in breadth, and the included cell was about 
half the size of the exterior, which it resembled in all its characters.” 
In the figure, however, the spines of the parent form are not shown 
on the offspring. I have repeated one of Professor Cleve’s figures 
(Plate I. fig. 1) on account of the interest it adds to my own 
observations. 
Many species of diatoms have been recorded with “internal 
valves ” as they are called, and it is generally supposed that they 
represent resting stages — or a provision against drying up — corre- 
sponding to a thickening of the membrane in ordinary cells. This, 
* Journ. Roy. Micr. Soc., October 1896. 
f Bihang till K. Svcnskci Vet. Akad. Handlingar , Bd. I. No. 13, p. 9, tab. 
1, fig. 3, a and b. 
