207 



of these liave considerable powers of attachment by the aid of 

 their broad locomotive discs ; their shells are excessively thick, 

 and though not so rounded in outline as the majority of the 

 littoral snails, yet they are not strictly elongate. An aberrant 

 snail ( Cladopoda arenaria) inhabits an irregularly convolute 

 tube agglutinated by the lower half to rock surfaces. The bi- 

 valves are very few in number, and the only commonly occur- 

 ring ones are My tikis rostratiis, M. Menkeanns, Modiola 

 atistralis^ which are anchored in patches or clusters by their 

 byssal threads ; amidst the mussels the little Lascpa ruhra finds 

 a safe dwelling. The rock oyster ( 0. glomerataj is occasionally 

 found here, out of its usual station below low-tide mark. On 

 flat rocky shores, as off Snapper Point, Aldinga Bay, and about 

 Stansbury and other places on the west side of the Grulf, the 

 cavernous depressions of the surface, and the shelter afforded 

 by the large flattened stones, are the lurking places of many 

 gastropods, whose shape of shell and formation of foot ill adapt 

 them to live in exposed situations. 



(b/ Those living in sand. This habitat is unstable under 

 the attacks of the waves, and no powers of attachment will 

 serve the animal ; safety lies in the quickness with which it can 

 penetrate the sand either to avoid being washed away in the 

 denudation of the deposit, or to force its way to the surface 

 through any accumulation that may he piled over it. The only 

 gastropod which habitually lives in this habitat is Natica coiica, 

 whose shell is smooth and subconic, not well adapted for bur- 

 rowing, but offers a passive resistance to wave currents. On 

 the ocean seaboard, where tidal disturbances are so much 

 greater, there lives anothes species {N. Incei), which has a hemi- 

 spheric shell. The bivalves, which inhabit this kind of deposit, 

 have compressed valves and hatchet-like or cylindrical forms 

 well suited for rapid penetration ; whereas the globular form 

 of shell, which is ill-adapted to force its way through a resist- 

 ing medium, is unrepresented on our sandy tidal region. One 

 of the most active in penetrating sand in which it lives is Soleii 

 vaginoides, having a narrow cylindrical form. All the bival res live 

 in long burrows, and have long siphons, which must be valuable 

 in self-defence, more especiallv in the case of their young, 

 which, from their small size, would be affected by much slighter 

 wave disturbances than the adults, and would be in constant 

 danger if obliged to live at the surface or close to the surface 

 of the deposit that gave them shelter. 



The little Mesodesma elongata resides at little depth below 

 the surface, and nearer high-water mark than any other of the 

 burrowing species, and is therefore particularly liable to be 

 exposed, as is fully attested by the vast numbers displaced 

 during gales. It burrows, however, very readily, and moves on 



