53 



two slips of glass, in some very fluid balsam, the portions can some- 

 times be separated while under the field of the microscope, by sliding 

 one piece of glass over the other. 



Now, with regard to the observations of others, connected with 

 this genus. I am informed by Mr. Henry Deane, that he has rea- 

 son to believe that some at least of the individuals are attached to 

 the Alga by a short pedicle, and, I believe, he considers it to be of a 

 siliceous nature. 



In all the specimens I have examined I am satisfied there is no 

 trace of such an appendage, but that they are all sessile, and the su- 

 perior and inferior valves correspond exactly in all respects. I can- 

 not but imagine, that the radiated structure of the central orifice 

 must have deceived Mr. Deane, in this point, possibly in con- 

 sequence of his not having viewed the valves, when detached from 

 each other, as opaque objects. 



I was informed by Mr. Marshall that he considered these shells 

 were disposed one above another in a sort of column, in a similar 

 manner to the shells of the Gallionella, but I have not been able to 

 find a single instance in which they were so placed. I scarcely 

 know that I am justified in recording this as Mr. Marshall's opinion, 

 and I question much if he now holds it. I only mention it, as it 

 brings me to the consideration of the mode of propagation, a point I 

 have not been able to arrive at. I have generally found these shells 

 scattered singly all over the Alga, but in one or two instances I fan- 

 cied I met with them double (that is, one on the other), or pro- 

 gressing towards that state. It is not improbable that the mem- 

 branes named as attached to the two annular valves may, possibly, 

 be extended and developed into valves similar to the discoid ones, 

 and thus propagation may be performed by self-division, as in 

 many of its congeners. 



There is one consideration unfavourable to this view, as also to 

 that at first adverted to, namely, that they are found of all ima- 

 ginable diameters from - 004 to '014 of an inch, scarcely any two 

 being precisely the same, though if the method of propagation 

 above mentioned were correct one would expect to find many in the 

 same spot alike. 



Another objection to this suggestion is, that the number of rays 

 in each individual is by no means constant, and in some of the speci- 

 mens there is, in one of the segments formed by the rays, an 

 evident tendency to divide, the rudiments of a new ray being very 



