Oct., 1907. New Cricoids — Slocom. 277 



Inter distichals (I D) : Any plates situated between the distichals. 



Anal Plate (X) : First plate of the anal interradius. It often rests 

 upon the posterior basal and is in line with radials or nearly so. 



Interbrachials or interradials (I Br) : Any plates of the interbrachial 

 areas. 



Radianal (RA) : A plate resting within the angle formed by two 

 basals and below the right posterior radial plate. It joins the anal 

 plate on the left and occupies the position of an inferradial. 



Distal: Farthest from the stem. 



Proximal: Nearest to the stem. 



Lateral: Pertaining to the side. 

 Authors differ widely in the use of the terms" mold," "cast" and "im- 

 pression," but as used in this paper they may be defined as follows: 



Natural mold: A matrix or cavity in the rock, formed, by natural 

 causes, around an organism when the rock was plastic. The external 

 form of the organism is thus preserved. 



Natural cast : The rock filling of the internal or visceral cavity of 

 an organism, the shell of the organism having served as a mold. In 

 many cases (see Plate LXXXIV, Figs, 3 and 10) the shell is afterwards 

 dissolved out, leaving the mold and cast in position. In such cases the 

 space between the two indicates the thickness of the shell. 



Impression: An artificial cast, made in a natural mold, which shows 

 the external features of the organism. 



In the following descriptions three materials are referred to as being 

 used for making impressions, viz. : plaster, roller composition, referred 

 to as "composition," and vulcanized rubber, referred to as "rubber." 

 The methods of making impressions from the two latter substances 

 have been previously described by the writer.* In addition to the 

 details there given it may be remarked that in many cases the shape 

 and position of the mold in the rock is such that it is difficult to keep the 

 rubber in place while vulcanizing. In such cases it is necessary to build 

 a retaining wall of plaster, or to cement a metal ring around the open- 

 ing of the mold ; this prevents the rubber from spreading when the 

 pressure is applied with the clamp, and a perfect impression is assured. 



* Science, 1907, X. S. Vol. XXV, p. 591. 



