BRACHIOPODA. bl 
examples of persistence of type, since forms very similar to the present 
day Discinas and Cranias occur in the Palaeozoic, Ordovician and 
Silurian strata. 
A piece of rock, with several specimens of Crania anomala 
attached, is exhibited. 
Section II. — Articulata. 
Order 3. Protremata. — This group, formerly very abundant, is 
now almost extinct, the Family Thecidiidce representing the Order at 
the present day. Thecidium medilerraneum (Fig. 9) forms little oval 
boxes about a third of an inch in length, shaped somewhat like a 
Thecidium mediterraneum. A, natural size. B, section through shell. 
Magnified. 
pear cut in half (peduncle valve), and with a semicircular lid 
(brachial valve) working on a hinge on the upper flat surface. The 
foramen and peduncle are absent ; but between the pointed end of 
the peduncle valve and the hinge is an area filled in by a calcareous 
plate characteristic of the Protremata. 
The brachial valve opens like the lid of a snuff-box, and shuts 
down on the least alarm with the rapidity of lightning. The 
peduncle valve is fixed on the rocks by its convex surface. The 
species is common in the Mediterranean in from 30 to 300 fathoms, 
and is also found in the West Indies. 
