106 THE LIVING WORLD. 



began to disappear and the modern type became abundant. The first 

 water and land snails are found in the Carboniferous (4). 



Pteropoda (winged mollusks). This is a little-known class, found 

 to-day chiefly in the high seas. Pteropods were abundant in the 

 Silurian (2) rocks, the genera seeming to be identical with those of 

 to-day. The group has remained practically unaltered till to-day, 

 except that some of the early families have entirely disappeared. At 

 no time has the class been of much importance in the world. 



Scaphopoda (tooth shells). This is also a small and unimportant 

 class, consisting to-day of only three genera. It is of great antiquity, 

 however. The first remains are found in the Devonian (3), although 

 the class probably existed long before that era. It has remained 

 practically stationary. 



Cephalopoda (squids, cuttle fishes, etc.). This is by far the highest 

 of the mollusks, and it has at all ages been an important group. The 

 class has two well marked orders, one having four gills ( Tctra- 

 branchiata), and the other two gills (Dibranchiata). Beginning 

 with the early Silurian (2), the Tetrabranchiata were quite abundant. 

 Fourteen orders were then in existence. They gradually increased 

 in abundance, size, and complexity, although only one new type was 

 introduced during the Devonian, and no others until the Tertiary (9). 

 They reached a very high state of development in the Mesozoic (8). 

 From that time they rapidly diminished, and to-day only a single 

 species (Nautilus) is left as a remnant of this once predominant 

 type. Nautilus is in itself interesting as an example of a long- 

 persistent species, being abundant as far back as the Silurian (2). 

 Just before the tetrabranchs reached their culmination the first repre- 

 sentative of the dibranchs appeared (Triassic). These have con- 

 tinued to increase until the present time, one large section of them, 

 however, disappearing with the Cretaceous (Belemitcs). To-day 

 the class, though existing in great numbers, is impoverished as to 

 variety, only a few types remaining. 



The mollusks in general will thus be seen to be a very old group, 

 just about as well differentiated at the beginning of the Silurian (2) 

 as they are to-day. The long ages have seen them always in abun- 

 dance, and have witnessed slow changes and slight progression. 

 There has been a constant expansion of the group, but it has consisted 

 in the multiplication of families and genera. As elsewhere, the be- 

 ginning of the Mesozoic (5-8) saw the older types giving way before 

 the modern ones. 



