164 BRACHIOPODA 



monly lives in burrows in the sand of the sea-floor, the 

 contraction of the muscles of the peduncle serves to with- 

 draw the animal from the surface into its burrow. 



The opening for the passage of the peduncle varies 

 considerably in different genera, and is a feature of 

 importance in classification. The simplest case is that 

 found in Lingula and some others, in which the opening 

 is shared by both valves. In other types we find that 

 the peduncle-opening is confined to the ventral valve ; 

 in Distinct the opening is completely enclosed by the shell 

 and is often near the centre of the valve, consequently the 

 peduncle comes out at right angles to the plane of the 

 valves. Sometimes, as in Orthis (fig. 80), the peduncle- 

 opening is in the form of a triangular fissure, under the 

 umbo, known as the delthyrium. In brachiopods belong- 

 ing to the group Telotremata, a delthyrium is found in 

 young individuals, but subsequently becomes partly closed 

 by two plates, which grow inwards from the sides of the 

 delthyrium and sometimes meet in the middle line. These 

 two plates form the deltidium (fig. 72 A, d). In Rhyn- 

 chonella the two plates usually meet, but a small circular 

 or ovate opening (the foramen) is left near the centre for 

 the peduncle. In Magellania (fig. 72 A, f) the foramen 

 is quite at the apex of the umbo, its lower boundary being 

 formed by the deltidium (d). In genera belonging to the 

 Protremata and a few of the Neotremata, the delthyrium 

 is more or less completely closed by a single plate known 

 as the pseudo-deltidium ; this at first sight closely resembles 

 the deltidium, but is really of a different nature. It 

 originates on the dorsal surface of the body, but subse- 

 quently becomes attached to the ventral valve, and then 

 continues to grow by secretion from the peduncle. The 

 deltidium, on the other hand, is formed by the edge of the 



