XVII 



COLOUR 467 



arranged in concentric circles on the ribbed shell. They are not 

 so long as the spines irregularly scattered on the shell of Rli. 

 spinosa from the Jurassic formations. Some shells are brightly 

 coloured, as, for instance, the various species of Cistflla which 

 live on the coralline rock in the Mediterranean ; thebo exhibit 

 bands or rays of alternate orange and bright pink. On the 

 other hand, the shells of Terehratida vitrea are of a slightly trans- 

 lucent white, and of the utmost delicacy. They are very large, 

 so that the cavity within the valves is of much greater size than 

 the body of the animal, but in other genera the soft parts are 

 packed very closely, and there is but a very small mantle-cavity 

 or space within the shell unoccupied, by the body of the animal. 

 It is, however, more common for the shells of Brachiopods to be 

 of a dull yellowish colour, and to be somewhat massive. ^lost 

 species are attached by a pedicle or 

 stalk to some rock or stone at the 

 bottom of the sea, but in some, as in 

 Crania^ the ventral valve becomes 

 closely adherent to its support, so 

 much so that it is difficult, or impos- 

 sible, to remove the animal without 

 leaving the ventral valve behind. 

 Linqula.Y[kQ (7rama, one of the Ecar- ^ .,.,.. „,. .,^fnr, 



'^ , , . Fig. 013. — ihree specimens 01 Gv'a- 



dines, lives in sand (Fig. 321, p. 483), Hiaonoma^o on a stoue dredged 



and does not use its long pedicle to ^^ ^och Fyne. .Tbe topmost 



o ^ specimen is seen in pronle. 



adhere to any fixed object. 



The outline of the shell varies extremely. It maybe almost 

 round or prolonged along either axis ; the edges of the valves 

 may be smooth or fluted in correspondence with the ridges and 

 grooves of the outside of the shell. 



On the inner surface of the shell of the Testicardinate Brachio- 

 poda, at the hinder end of the ventral valve, are two lateral 

 teeth, w^hich fit into corresponding sockets in the dorsal valve. 

 These form a hinge, which in many cases is so arranged as to 

 permit the shell to be opened to only a very limited extent. 

 There are also certain plate-like processes which project into the 

 lumen of the shell, and help to support various portions of 

 the body; and in Terehratida^ Waldhehnia^ etc., these form a 

 complicated band-like loop, which increases in complexity with 

 advancing age, and serves to support the arms. In the extinct 



