Many shells, bones, leaves, and other forms of organic matter are
preserved as molds and casts. If a shell had been pressed down into the
ocean bottom before the sediment had hardened into rock, it may have
left the impression of the exterior of the shell. This impression is
known as a _mold_ (Pl. 2). If at some later time this mold was filled
with another material, this produced a _cast_. This cast will show the
original external characteristics of the shell. Such objects are called
_external molds_ if they show the external features of the hard parts
(Pl. 2, fig. 2) and _internal molds_ (Pl. 2, fig. 3) if the nature of
the inner parts is shown.
Molds and casts are to be found in almost all of the fossil-bearing
rocks of Texas, and they make up a large part of most fossil
collections. It is particularly common to find fossil clams and snails
preserved by this method. This is primarily because their shells are
composed of minerals that are relatively easy to dissolve, and the
original shell material is often destroyed.
[Illustration: PLATE 3
Silicified Brachiopods
All specimens from Permian limestones of the Glass Mountains, Brewster
County, Texas]
Figures—
1, 2. _Avonia_ sp., ×2. Ventral and side view of two pedicle valves
showing long slender spines.
3. _Avonia_ sp., ×6. Young specimen showing attachment ring at apex.
4-6. _Muirwoodia multistriatus_ Meek, ×4. Respectively, side and
ventral view of pedicle valve and dorsal view of brachial
valve.
7-9. _“Marginifera” opima_ Girty. Respectively, ventral and side
view of pedicle valve showing long stout spines (×4) and
interior of brachial valve showing muscle scars and brachial
ridges (×2).
10-13. _Aulosteges tuberculatus_ R. E. King, ×4. Respectively, side
and interior view of brachial valve showing muscle scars;
ventral view of pedicle valve showing brush of attachment
spines on ears; and ventral view of a young pedicle valve.
14. _Avonia_ sp., ×4. Ventral view of a specimen with long spines.
15, 16. _Avonia subhorrida_ (Meek), ×2. Ventral view of a pedicle
valve and dorsal view of a brachial valve showing spines on
both.
17. _Avonia signata_ (Girty), ×2. Dorsal view of a large specimen
showing hairlike spines on brachial valve.
18-20. _Prorichthofenia permiana_ (Shumard). Respectively, side and
posterior view of pedicle valve (×4) and interior of dorsal
valve (×2) showing anchor spines and interior spines of the
brachial valve.
21. _Heteralosia hystricula_ (Girty), ×2. Cluster of individuals
attached to a large _Marginifera_.
Photograph courtesy of Dr. G. A. Cooper, U. S. National Museum.