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 equi-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 Lingula 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 1 02. The Terebratulidae are best represented. There were 

 once 300 or 400 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 shell? 



* " Woodward's Manual," p. 135. 



