138 ON THE COLLECTION AND 



Fig. 3. — Pyxicola annulata : o., operculum ; c.v., contractile vesicle. 



J 5 4. — Platycola hicolor, showing h., hood ; ce., oesophagus; c, collar. 



,, 5. — The ordinary shape of the lorica of P. Ucolor, showing 

 colourless collar. 



6. — P. hicolor enlarged, the letters as before. 



7. — An abnormally elongated specimen of P. hicolor Q.). 



8. — Platycola aiirita. 



9. — Lorica of P. aurita viewed from above. 





®\\ tbc Collection anb preparation of 



tbe ©iatontace^* 



■ By Alfred W. Griffin. 



Part I. — Collection. 



I CANNOT claim for this paper any originality of thought, 

 neither can I introduce anything particularly novel ; it is 

 simply an attempt to gather together some of the ideas of 

 the best authorities on the question, for the benefit of those whose 

 want of leisure precludes them from searching out these facts for 

 themselves. The study of the life-history of the Diatomace^e is 

 in itself a stupendous work, and the interest excited by it is 

 increasingly great. But far more interesting is the study of their 

 siliceous framework, which alike resists time and decay. The 

 softer part of the frustule is requisitioned by Nature, and utilised 

 by her in her many and varied operations, in obedience to that 

 law which compels organic matter to undergo constant changes, 

 whether progressive or retrogade in its work of utility. 



The animated matter quickly loses its identity and its relation 

 to form, but the silicified skeletons of Tricerathmi and Cosci?io- 

 discus are as perfectly preserved in the mud of the Thames as in 

 the various fossil deposits of California, or those found in the 

 guano of South America. There is scarcely a species of 

 Diatomacese that occurs in this quasi-fossil condition which has 

 not its living representative. 



The .^gina clay marl, which is without doubt the oldest 



