572 CONCHIFERA. 



equal, symmetrical valves united by a hinge, without a liga- 

 ment, and opened and closed by means of special muscles 

 which are attached to their inner surface, the office of the 

 oral arms being confined to procuring and detaining their 

 food. As these animals lie upon their backs, the lower 

 valve is properly dorsal with reference to the body, while 

 the upper, convex, and perforated valve must be regarded as 

 ventral. In the Terebratulidm the valves of the shell are 

 calcareous and finely perforated, the tubular apertures 

 being lined in the living animal with prolongations of the 

 mantle ; in the Rhynchonellidte the structure of "the shell 

 is horny and imperforate; in the LingulidmxX is thin and 

 covered with an epidermis ; in the Orbiculidte it is nearly 

 flexible and almost entirely horny; while the shells of 

 the fossil Productidie are generally spinulose; in the 

 extinct tribe Radiolitida the texture of the lower valve 

 is cellular; and in the Hippuritida the upper valve is 

 pierced with pores which branch off towards the surface. 



The characters of the various genera are principally taken 

 from modifications of the peculiar, bony apophysary sys- 

 tem, which is situated in the lower or dorsal valve, and 

 which serves to support the cirrhated oral arms; two slen- 

 der processes usually proceed from near the hinge-margin 

 of the smaller imperforate valve and form loops or simple 

 apophyses, and sometimes there is a central perpendicular 

 lamina between them, to which the crura of the lateral pro- 

 cesses are attached, either singly or doubly, and constitute 

 a very complicated apparatus. 



The existing species of Brachiopods are found in all lati- 

 tudes, and appear to be most varied and abundant at consi- 

 derable depths ; their numbers, however, are few when 

 compared ^jvith those of their fossil prototypes, which for- 



