62 RELICS OF PRIMEVAL LIFE 



probable it was, a mollusk, it must have been one 

 of advanced type, and with a highly complex struc- 

 ture, as well as the singular apparatus for flotation 

 implied in a chambered shell with a siphuncle. 



Next to this among these primitive Mollusks are 

 straight and spiral shells representing those delicate 

 and beautiful animals of the modern seas, the 

 Pteropods, or wing-footed Sea-snails, beautiful and 

 graceful creatures, the butterflies of the sea, and 

 moving in the water with the greatest ease and 

 beauty by the aid of membranous fins, or wings, 

 sometimes brightly coloured. These creatures 

 abound in all latitudes in the modern ocean, and 

 their delicate shells sometimes accumulate in beds 

 of " Pteropod sand." They very early entered on 

 the arena of marine life, and have continued to this 

 day. 



We miss here the two great Molluscan groups of 

 the creeping Sea-snails like the limpet and whelk, 

 and of the ordinary bivalves like the oyster and 

 cockle. Both are present in the lowest Cambrian, 

 though in small numbers compared with their 

 present abundance. Possibly they had not yet ap- 

 peared in the Etcheminian Sea, though the muddy 

 and sandy bottoms, evidenced by its slates and 



