TRIASSIC PERIOD 73 



archaic character, but fresh threads, so to speak, were BIVALVED 

 coming out of the tangle. Some forms in the last Age had MOLLUSCS 

 been developing in the direction of mussels ; and fresh- 

 water and sea mussels of modern type were now in being 

 (Unio, Mytilus). New forms of ark-shells were also in view ; 

 some of which are still represented in the mollusc world 

 (Limopsidce). Oysters somewhat of the " thorny " kind 

 were also on the scene (Plicatula) ; and cockles, which had 

 more or less given sign in the last Age, were now rid of old 

 partnerships, and established as a distinct family (Cardiida). 

 Among the bivalves with long, retractile siphons, new forms 

 were to be seen (Corbula). These appear to have been of the 

 type of the " gaper " molluscs of our own time (My ado), 

 with " feet " too large for their shells. 



Gastropods fell far short of the bivalved molluscs in point UNIVALVED 

 of number : and it was a very long time before they equalled, MOLLUSCS 

 and finally surpassed them. The mental superiority of 

 snails to bivalves may not have been so great then as 

 now. 



The most noteworthy fact in the Triassic annals of gastro- 

 pods was an elaboration of the breathing system in certain 

 species. Most of the marine snails then living probably 

 breathed entirely through the skin, as do some of the 

 fraternity at the present time. Some forms possessed gills 

 as additional means of respiration ; and it was in the 

 case of certain of the gilled snails that notable changes 

 had taken place. Aerated water had, of course, to be carried 

 to the fold in the mantle where the gills were located ; and 

 this had hitherto been done through the agency of modified 

 edges of the mantle. Under the new arrangement the work 

 was being done in a more cleanly and efficient manner by 

 means of a well-defined tube or siphon (Cerithiidce). 



In spite of the multiplications and deployments of bivalves, 

 and the on-coming of univalves, cephalopods continued the 

 leading molluscs. Their adaptive powers were certainly 

 great, and they were probably the " intellectuals " of the 

 mollusc world. How far their mental development was 

 accelerated by the habit of walking more or less on the head 

 cannot be determined. 



