XVII COLOUR 467 



arranged in concentric circles on the ril)bed shell. They are not 

 so long- as the spines irregularly scattered on the shell of lili. 

 spiiwsa from tlie Jurassic forniations. Some shells are brightly 

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

 live on the coralline rock in tlie Mediterranean ; these exhibit 

 i)a.nds or rays of alternate orange and briglit pink. On the 

 other hand, the shells of Terelwatula mlrea are t)f a slightly trans- 

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

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

 the body of the animal, but in otlier genera the S(^ft 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 Urachiopods to l)e 

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

 species are attached by a pedicle or 



stalk to some rock or stone at the ^--ss^^^^^^ 



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 dihicult, or impos- 

 sible, to remove the animal without £ 

 leaving the ventral valve behind. ^ ^'- -^ 

 Lin.ijnla., like Crania, one of the 

 Ecardines, lives in sand (Fig. 3 2 1. ^i«- '^^^.-Three specimens of Cm- 



^ ;- • ma anomala on a stone aredf^ed 



p. 483), and does not use its long in Loch Fyne. The topmost 



pedicle to adhere to any fixed ol)ject.^ •^i"^"'"'^" ^' ''^"^ '"^ i^™^^"- 



The outline of the shell varies extremely. It may be almost 

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

 may be sniocjth or fluted in correspondence with the ridges and 

 grooves of tlie outside of the shell. 



(3n tlie inner surface of the shell of the Testicardinate Brachio- 

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

 teeth, which fit into corresponding s(jckets in the dorsal valve. 

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

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

 There are also certain ])latc-l!k(! ])rocesses whicli ]irojcct into the 

 lumen of the shell, and hcl]) to sup])ort various portions of 

 till', body ; and in TrrcJimt ula , WahlJiriinia, etc., these Ibrm a 

 complicated band-like looj), whicli increases in complexity with 

 advancing age, and serves t(j suppoi't the arms. In the extinct 



