INTRODUCTION. 15 



The time of exposure, even with tlie aperture in the diajihragm of the 

 condenser quite small, is very short, for magnifications of about 100 only about 

 one second, for magnification of 1800, 15-30 seconds. 



I must record my obligations to the late Mr. Alexander Agassiz and to the 

 Smithsonian Institution for placing this large and interesting collection of 

 sjjonges at my disposal; and to Mr. R. Kirkpatrick of the British Museum 

 and Mr. L. M. Lambe of the Geological Survey of Canada, for the tyjies they 

 sent me for examination and comparison. 



With all this material and with all my energy and experience, however, 

 this report could not have been produced had not Mr. Agassiz supi^lied the 

 costly apparatus necessary and in every way furthered the work with so 

 generous a liberality. 



The method of microphotography with ultraviolet light, employed in this 

 monograph for the delineation of the microscleres, will, I do not doubt, when 

 utilized more generally for the representation of minute structures of this kin^l, 

 increase the exactitude, and thus raise the standard of descriptive zoology; and 

 am most grateful to Mr. Agassiz for enabling me to introduce the method into 

 this branch of science and thus to assist in its advancement. 



