16 PAL/KONTOLOGY 



or hinge-plates, no crura or loop ; in fact, the only 

 internal features of the shell are the muscle-impres- 

 sions and a small median septum in the ventral valve 

 anterior to the delthyrium. Here is a brachiopod differ- 

 ing greatly from all the previous forms, especially in not 

 having its valves hinged together, but only attached by 

 muscles and other soft tissues. There are six species 

 of Discinisca living and one species of the allied genus 

 Discina : they may be spoken of as discinids. Many 

 fossil discinids are known, but owing to the inarticulate 

 (unhinged) character of the shell it is commoner to find 

 isolated valves than complete shells the reverse is the 

 case in terebratuloids and rhynchonellids, and indeed 

 most brachiopods. 



Lingula anatina (Fig. 7, b) is a brachiopod which 

 burrows in the sea-bottom in very shallow water; it 

 has a very long pedicle which it uses to draw itself 

 into its burrow when necessary. Like Discinisca it has 

 a horny shell. In shape it is somewhat oblong, the two 

 valves being very nearly alike, each having the umbo at 

 the extreme posterior end. There is no delthyrium, the 

 valves diverging at the posterior end for the passage 

 of the pedicle. Internally the valves show no hinge- 

 structures or brachial skeleton, though the spiral brachia 

 are present as usual ; but in the dorsal valve there is just 

 below the umbo a reflected portion of the shell resembling 

 a cardinal area. The muscular impressions are faint, 

 but are more complex than in previous cases, the absence 

 of articulation making it possible to have muscles for 

 sliding the valves sideways as well as for opening and 



