14 BUSK, ON THE STRUCTURE AND 



its characters are so well marked that we need not wait till it 

 shall be found in abundance, as it probably will some day. 

 It would, however, certainly have been overlooked in Loch- 

 leven, but for the minute search to which the gathering was 

 subjected. The same remarks apply to Navicula lacustris and 

 to Navicula lepida. 



Whenever, therefore, a gathering is met with which appears 

 to contain a great variety of forms, like the four above men- 

 tioned, it should be systematically and minutely searched, 

 and any striking forms, no matter how scarce, noted and 

 figured. If true species, they will most probably be found in 

 greater abundance elsewhere. 



It is much to be regretted that no work yet published con- 

 tains figures of all the known species or forms named as 

 species by their observers. Even in Ehrenberg's last great 

 work, in which many hundred species are figured, I observe 

 the names of about 350 species, most of which are described 

 as remarkable or characteristic of certain localities, not one of 

 which is figured, although most of the common species are 

 many times represented. 



Supposing, then, that all those forms which I have just 

 described as new to science should prove to be good and 

 distinct species, of which I cannot, of course, be sure, it is out 

 of my power to ascertain whether they may not agree with 

 some of the species named, but not figured, in his last work, 

 by Ehrenberg. I ought to mention, however, that several of 

 the species of my first section, new to Britain only, were con- 

 sidered by myself and others as new to science, till I found 

 them figured in Ehrenberg's ' Microgeologie,' when of course 

 I adopted his names for them. 



An Account of the Structure and Relations of Sagitta 

 BIPUNCTATA. By G. BusK, F.R.S. 



The minute creature to which the above name has been 

 given, though abundant, perhaps, in all seas, and noticed so 

 long ago as in the year 1781, has received but little attention 

 from zoologists in general. Its curious and interesting struc- 

 ture, however, and doubtful position in the animal kingdom, 

 render it a subject well worthy of further research ; and its 

 minute size, and the extreme delicacy and transparence of its 

 tissues, make it peculiarly an object of microscopical inves- 

 tigation. Though perhaps unknown, even by sight, to many 

 of our readers, the Sagitta bipunctata will probably be met 

 with on every part of the coast ; and it may be procured, 



