eres 007. NEw CRINOIDS — SLOCOM. a7 
Interdistichals (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 asamold. In 
many cases (see Plate LX XXIV, Figs, 3 and 10) the shell is afterwards 
dissolved out, leaving the mold and castin position. In such cases the 
space between the two indicates the thickness of the shell. 
Impression: An artificial cast, made ina 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, anda perfect impression is assured. 
Pierence. 1907,,N. 0. Vol. X XV, p. sor: 
