1868.] 361 [Edwards. 



upon them for fbrt\'-five days longer. In shad tlie eggs require a 

 temperature of 75°, are hatched in sixty hours and have a pendant 

 yolk sac for only forty-five to sixty hours. 



Mr. Alplieus Hyatt presented a copy of No. 5 of the Bul- 

 letin of the Museum of Comparative Zoology, published 

 about a week since, but accidentally bearing no date of pub- 

 lication. In it he had attempted to group the numerous 

 species of Ammonites into natural genera and families. 



Section of Microscopy. January 8, 1868, 

 Mr. John Cummings in the chair. Ten members present. 

 The following paper was read : — 



Note on a point in the Habits of the Diatomace^ and 

 Desmidiace^. By Arthur Mead Edwards. 



Although most writers on the suliject are in the habit of stating 

 that many of the genera of DIatomaceas in the living state are free, 

 or non-adherent to other larger algaj or submerged substances, yet 

 always since I first began the study of the Protophytes, as is well 

 known to my fellow- students with whom I have from time to time 

 discussed the subject, I have held that all species are, at some period 

 of their existence in an adherent or attached condition, growing upon, 

 for the most part, aquatic vegetation of a larger size. I have also 

 fi'equently expressed the opinion that the adherent condition of any 

 species was but temporaiy and conditional ; otherwise I could not see 

 how the wide distribution of forms, such as Cocconeis scutelluin, an 

 extremely widely diffused marine species usually found attached to 

 larger algae, or Tabellaria flocculosa, an equally cosmopolitan fresh 

 water species found almost invariably attached, was provided for, as 

 no motile spores of any kind are known to exist in this family, 

 although such may be the case. 



At the outset of my studies of tliese extremely interesting organ- 

 isms I naturally accepted the classification laid before me by the 

 authorities on the subject, and i-eferi-ed the forms I found to one or the 

 other of the divisions of free or attached genera, and, in fact, went 



