PLATE L. 



Figure 1. Skull of Erpetosaurus tabulatus Cope, showing a perfect 

 application of the method of using - the ammonium chloride deposit. The 

 effect had been heightened by the application of a white wash to the 

 background of matrix. It will be clearly seen that all of the details of 

 the skull structure are evident. This method is especially useful in study- 

 ing the course of the lateral line canals on the small amphibian skulls. 

 The supraorbital canals are thus indicated as shadows above the orbits 

 and were not previously observed. 



Figure 2. Skull of Erpetosaurus Isevis Moodie, showing the adapta- 

 bility of the method to irregular surfaces. The membra'nous skull bones 

 had become loosened from this cast and left only their impressions. The 

 block of coal is much broken, yet there are many details which are 

 evident on close study. 



Figure 3. An example of an imperfect fossil, representing a form 

 identified by Cope as Molgophis wheatleyi and showing an improper 

 use of the ammonium chloride deposit. The upper part of the fossil is 

 not covered uniformly with the rest, and will illustrate the advantage of 

 the ammonium chloride deposit as an illuminant for dark surfaces. 



Figure 4. The fossil is a portion of the skeleton of Ptyonius nummifer 

 Cope, and illustrates the disadvantages of the deposition of too much 

 ammonium chloride on the specimen. On removing the deposit by breath- 

 ing on it and making another more tenuous deposit the fossil was clearly 

 evident. 



