﻿2l8 NEW YORK STATE MUSEUM 



and draw them with a camera lucida but, while enough could be seen 

 to show that former analyses were certainly in error, yet lines 

 seemed to be absent where sutures were expected and faint or frag- 

 mentary lines suggested sutures where they were not expected. 

 Remembering a former successful showing of a vertical section of 

 the basals of Blastoidocrinus by photographing a portion of Billings's 

 type under a mounting of gum dammar (see plate 4, fig. 2) a similar 

 trial was made with the damaged plates of P. s t r i a t u s. A drop of 

 pure benzol was placed on the area in question to exclude air bubbles 

 and a drop of gum dammar, dissolved in benzol, added. A little of 

 the dissolved gum was also placed on an ordinary round cover glass 

 for microscope slides and the cover placed over the area. This was 

 allowed to dry enough to retain its position and more of the mount- 

 ing fluid then added in order to show as large an area as possible. 

 After a second partial drying the region was photographed with a 

 three inch objective, using a black hood over the lens to avoid reflec- 

 tions and with bellows extended to give an enlargement of ten 

 diameters. 



Advantages of the method. The photographs for figures 2 

 and 4 of plate 5 were made in this manner and a comparison of 

 them with figures 1 and 3 of the same plate (from photographs 

 made without the mounting) will reveal several peculiar advantages 

 of this method. Reflection from the summits of plate granules, tool 

 marks, scratches and small crystalline faces of broken calcite have 

 been reduced to a minimum and much of their distracting influence 

 on the eye removed. Refraction has relieved the surface shadows 

 cast by minor elevations and has also admitted light to the deeper 

 features of plate detail, making structures visible that were hereto- 

 fore obscured or lost. On the more uniformly lighted surface so 

 produced the difference in the amount of free carbon now held by 

 the calcite is quite clearly revealed and the former presence of or- 

 ganic membranes made manifest. It will be seen that the sutures 

 stand out clearly as black lines of uniform width and all of the 

 sutures of the damaged area were not only thus made clearly visible 

 under the microscope but were secured on negatives. The photo- 

 graphs for other figures were also prepared in this manner and the 

 features so revealed will be described in their proper places. 



Nature of the plate ridges. The figures on plate 5 represent 

 two different areas of the specimen and show the ridges as they 

 appear on IBB, BB and RR. They are seen to be arranged in groups 

 the members of which are rather evenly spaced, of regularly varying 



