34 HALF-AN-HOUR 



tions round will firmly fix the object, when another coat of the 

 Brown Cement will make the slide secure. Great care should be 

 taken that all needles, section-lifters, etc., are quite clean from one 

 fluid before being placed in another ; and on no account must 

 the rod or brush of one re-agent bottle be allowed to touch 

 another re-agent. 



1balf^an===1bour at tbe flfticro6cope 



Mltb /Iftr. XTutfen Mest, 3f.X.S., ff.lR./ID.S., 6lc. 



Plates 3, 4, 5, 6, 7, 8. 



Plagiogramma. — A name first given to a species of diatoms in 

 1859, in a paper which appeared in the "Quar. Jour. Micro. Sci." 

 in July of that year (p. 207). Ehrenberg appears to have pre- 

 viously described some-forms under the name of '''' Hetei'omphala)^ 

 this name it would have been desirable to retain. Even Ralfs (in 

 Pritchard) doubts the propriety of forming a new genus for it. It 

 comes near to Fragilaria^ Odontiiim^ and De?iticula: forming short 

 filaments, probably attached to their entire base, at least in their 

 early state. The first specimen which led Dr. Greville to form a 

 new genus for its reception, was found by the late Dr. Gregory in 

 dredgings from the Clyde, and described by him with figures in a 

 remarkable paper on that subject. It is now dedicated to him 

 under the name P. Gregoria7mm. Thirteen so-called species are 

 known, with two doubtful ones ; they mostly run very close, in 

 several instances single examples only were met with, and I am 

 persuaded the numbers will eventually have to be much reduced. 

 P. elongatum does not appear amongst those enumerated by Ralfs 

 (loc. cit.)^ from some of which, however, it scarcely differs. It 

 would be desirable to know where descriptions of this form are to 

 be met with. 



Most of Dr. Greville's forms were found in material ob- 

 tained from the Caribbean Sea; he says in a note (p. 207) : — "I 



