BEACHIOPODA. 



81 



examples of persistence of type, since forms very similar to the present 

 day Discinas and Cranias occnr in the Palaeozoic Ordovician and 

 Silurian strata. 



A piece of rock, with several specimens of Grama anomala 

 attached, is exhibited. 



Section II. — Aeticulata. 



Order 3. Protbemata. — This group, formerly very abundant, is 

 now almost extinct, the Family Theckliklce, representing the Order at 

 the present day. Thmdium mediierraiieum (Fig. 9) forms little oval 

 boxes about a third of an inch in length, shaped somewhat like a 



Fi?. 9. 



Thecidium mediterraneum. 



A, natural size. 

 Maguified. 



B, section 'througla" shell. 



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 aii 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. 



