Xll INTRODUCTION. 



conferred upon such forms by the presence of these accessory 

 methods of attachment and adhesion ; while the filamentous 

 species, being usually aggregated in considerable masses, or 

 entangled amidst the branches of the larger Algae, are also less 

 liable to dispersion. 



From an examination of the figures referred to in the above 

 list, it will be seen that while the general character of the 

 process, which consists in the intermingling of the endochrome 

 or cell-contents of the parent frustules, is the same throughout, 

 there are peculiarities attending the process in the different 

 genera which prevent us from giving a detailed description 

 of the phenomena that will apply to every case. The most 

 notable of these peculiarities are the variations in the number 

 of the conjugating frustules and the sporangia which result from 

 the process. These variations may be reduced to four classes. 



I. We have two parent frustules and two sporangia as the 

 result of their conjugation. This mode is seen in Epithemia, 

 Cocconema, Gompltonema, Encyonema, and Cottetonema. 



II. From the conjugation of two parent frustules we have a 

 single sporangium. This occurs in Himantidium. 



III. The valves of a single frustule separate, the contents set 

 free rapidly increase in bulk, and finally become condensed into 

 a single sporangium. This may be seen in Cocconeis *, Cyclo- 

 tella, Mehbira, Orthosira, and Schizonema. 



* I have not attempted to reduce the examples of conjugation given by 

 Dr. Griffith and Mr. Carter to any of the above classes. In the details and 

 figures of these writers there are circumstances which I cannot explain in 

 accordance with the known phsenomena attending conjugation in other cases, 

 and the significance of which I cannot comprehend without a personal exami- 

 nation of specimens. 



Judging from the figures given by Dr. Griffith of the conjugation of 

 Navicula, I should refer that genus to the third mode I have mentioned ; but 

 the text of Dr. Griffith's paper, speaking of "parent frustules," would point 

 to my second mode as the process he had observed. The " rugose siliceous 

 sheath " enveloping the sporangium, as described by Dr. Griffith, I have 

 never observed ; probably it may have been an appearance resulting from the 

 condensation and corrugation of the mucus developed around the reproductive 

 body. If the figure given in the ' Micrographic Dictionary ' of the con- 

 jugation of Surirella be correct, that genus will also fall under my second class. 



In Mr. Carter's paper, the formation of two sporangia in Cymbella and 



