407 
CHAPTER XIII. 
BRACHIOPODA. 
WHILE it is out of the province of this work to enter into any lengthy 
arguments as to the position of this class; while we treat of them 
here immediately after the Conchifera, it is not in ignorance that they 
would be more justly placed among the Molluscoida, probably very 
near to the Tunicata. They differ from ordinary bivalves in being 
always equal sided, but never quite egui-valved. Their valves are 
respectively dorsal and ventral. The ventral valve is usually largest, 
and has often a prominent beak, by which it is attached, or through 
which the organ of adhesion passes. The dorsal valve is always the 
smaller, and is free and imperforate. The valves are articulated by 
two curved teeth, which are so complete that the valves cannot be 
separated without injury. A few genera have no hinge. In Crania 
and Discina the lower valve is flat ; the upper like a limpet ; while 
the valves of Zinguda are nearly equal, and have been compared to a 
duck’s bill. In the Conchifera the sliding of the valves is well 
guarded against by means of hinges with teeth and sockets; but in 
the Brachiopods the same end is apparently attained by means of 
muscles. The blood system is not very complex, and does not differ 
very greatly from the same system in the Tunicates. The Brachio- 
pods are all natives of the sea; but little is known as to their 
development. Of all mollusca they enjoy the greatest range, both of 
climate, of depth, and time. A large number of the genera contain 
only extinct forms ; indeed of the 1,842* species formerly known, a 
few types of but a small number of genera only are left, numbering 
in all 102. The Terebratulidz are best represented. There were 
once 300 or.40o species of this family ; there are now not more than 
sixty-seven in the seas of the world. The difference between the 
past and the present is especially striking, when we compare the 
recent and fossil species of Europe. Among no other class of shells 
* ‘ Woodward’s Manual,” p. 135. 
