64 



C. HEDLEY. 



tide. It has a smooth, square, compressed carapace about 

 two inches broad and painted with transverse bars of white 

 and red. Another commoQ crab of about the same size is 

 Plagusia chabrus (fig. 29), which has the limbs and body 

 bristling with sharp thorns. 



Fig. 29. Plagusia chabrus, a common crab. 



The shells of those molluscs which are habitually exposed 

 are extremely solid. For instance Monodonta which lives 

 high on the beach has a far thicker shell than its relation 

 Calliostoma which lives entirely submerged. Among naked 

 molluscs Onchidium, which leaves the water, has a thicker 

 cuticle than the Nudibranchs which never come into the air. 



Below the Melaraphe zone the fauna rapidly increases. 

 The limpet, Helcioniscus variegatus, appears accompanied 

 by abundant gasteropoda: Nerita melanotragus, Monodonta 

 zebra, Drupa marginalba and Bembicium melanastoma; 

 Sypharochiton pellisserpentis represents another molluscan 

 division, the Polyplacophora. 



Dominant in the mid-tidal zone is the gregarious annelid, 

 Galeolaria ccespitosa. 1 The tube of this is small, white, 



1 Bush, Harriman Alaska Expedition, xii, 1904, p. 177. Haswell, Proc. 

 Linn. Soc. N.S.W., ix, 1885, p. 665, pi. 31, fig. 5, pi. 32, fig. 1, 2. 



